Devices: experimental results and discussions

Room-temperature (RT) PL spectrum for the 1.55-μm sample of “MD3QW” is shown in Fig. 4.6(a). The PL emission peak is located at λ = 1540 nm with a full-width at half maximum (FWHM) of 100 nm (1468-1568, nm). The emission peak is close to the calculated e1-hh1 transition at λ = 1550 nm. In Fig. 4.6(b), the FWHM of EL spectra is increased from 118 nm to 178 nm (1410-1588, nm) as injection current increased from 10mA (current density ~ 20 A-cm^-2) to 100mA (200 A-cm^-2). The RT-EL spectra were measured from the top of the mesa diode. Therefore, the EL emissions are TE-polarized dominated by the transitions between electron subband and heavy-hole subband. According to our calculation, the EL emission peaks correspond to e1-hh2 (λ = 1460 nm) and e1-hh1 (λ = 1550 nm) transitions within the QWs. The spontaneous emission intensity from e1-hh2 transition becomes stronger than the e1-hh1 transition when injecting current density above 20 A-cm^-2. The higher optical gain of e1-hh2 transition is explained by the increase of spontaneous emission rate for band filling effect in modulation-doped QWs [4.3], [4.4].

On the other hand, once the ridge-waveguide laser diodes are fabricated, in first, they should be qualified by current vs. voltage (I-V) measurement as demonstrated in Fig. 4.7.

Comparing with the ideal diode, the real diode indeed has few leakage currents from the diode junction at low forward applied biases. Also, for large forward biases, there is a saturation effect as shown by the dashed line. This is due to ohmic losses from the finite resistance of the neutral n- and p-regions [4.5]. Generally speaking, in condition of serious resistance (R) < 10 Ω and slope factor (nf) < 2, diode performance is good enough to us for its further light emission test. The ridge-waveguide lasers are measured under continuous-wave (CW) operation at 20^oC controlled by a thermoelectric (TE) cooler. Fig.

4.8(a) and (b) separately shows the Light vs. current (L-I) characteristics and the lasing spectra for the FP-laser of waveguide width = 2.2 μm and length = 930 μm. The threshold current (I ) is ~ 60 mA, and the slope efficiency is ~ 47 μW/mA per facet. No kinks are

observed in the L-I curve. The laser spectra in Fig. 4.8(b) show an intriguing emission performance. At I = 70 mA, the laser spectrum shows a single lasing peak at λ = 1514 nm.

As we increase I = 100 mA, in addition to the main peak at λ = 1514 nm, a second lasing peak at λ = 1528 nm is observed. Further increasing the injection current up to I = 130 mA, the laser spectrum exhibits three peaks at 1514, 1528 and 1545 nm.

The laser performance of the TEF-laser is shown in Fig. 4.9. In comparison with the FP-laser ones, the I_th for the TEF laser is increased to 100 mA. And, the slope efficiency is ~ 42 μW/mA per facet. The increase of Ith and the decrease of slope efficiency are caused by the reduced mirror reflection for the tilted facet of θ = 7^o. The TEF-laser spectra at injection current above threshold are shown in Fig. 4.9(b). However, the TEF-laser maintains only single lasing peak operation at λ = 1511 nm, even that the injection current is up to 120 mA.

The FP laser performance has two notable signatures: (1) lasing starts at excited energy state of λ = 1514 nm; (2) lasing peaks sequentially appear from λ = 1514 nm, 1528 nm, to 1545 nm as injection current increases. These transitions are not either predicted by the subband calculation for the triple QWs structure, or identified in the RT-PL and RT-EL spectra.

To discuss how possibilities could cause this three unusual peaks emissions from FP-laser and why they start from the shorter wavelength, following discussions are given:

1) We use a new quantum mechanics tool, named as nextnano³ [4.6], to calculate whole laser structure in one-dimension with considering n-type doping inside the tensile-strained InGaAlAs barrier. Also, Schrödinger-Poisson equations, current continuity equations, and the current relations for electrons and holes are involved. When bias = 1.0 volt and current density = 3 A-cm^-2, the simulation results show the band diagram has already been in flat-band condition. It also shows the quasi-Fermi level of electron (Efn) is already 239meV above e1, however, the quasi-Fermi level of hole (E_fp) is still below the hh1-state about 3meV. Moreover, according to our voltage-current measurement of this

FP-laser diode, bias over 1.4 volt could be possible to offer the injection current density higher than threshold current density (Jth ~ 3K A-cm^-2). To say simply, the current density for lasing at J_th should be 1,000 times higher than that existing in flat-band condition. In order words, the electron-1 subband has been totally population inversions at its threshold.

It also means that optical gain will be dominated at how many holes are in condition of population inversions. Therefore, we deny to concern these multi-wavelength stimulated emissions could be possible caused by operating QWs under different and non-uniform built-in electrical field owing to n-type MD.

2) Then, we further consider the coupling effect between QW’s if possible to cause a splitting between electron- and heavy-hole subband higher than 7.5meV (1514-1528, nm) or 8.9meV (1528-1545, nm). In the case of two-period QW’s separated by 8.6-nm tensile-strained InGaAlAs barrier, the calculation results show they exist very weak coupling. And, it induces only 0.6 - 0.7meV separation between the transitions of e1^even-hh1^even and e1^odd-hh1^odd. Again, we don’t have to take coupling-effect into account.

3) When we further look back on the materials what we grew and the growth process, these thin compressive/tensile strain InGa(Al)As alloys could be possible to form and mix localized states like as quantum wires/dots due to the relative low growth temperature and In-Ga segregation/conjugation. To investigate the detailed electron-hole transitions in the epi-structure, photocurrent spectroscopy was carried out. The photocurrent spectrum at zero bias is shown in Fig. 4.10. The spectrum shows δ-like absorption peaks at λ = 1514 nm, 1528 nm, 1545 nm, and 1560 nm. Normally photocurrent spectrum features the joint density of states of the transition levels. Step-like signals are expected for absorption transitions of quantum well states, while the δ-like absorption peaks correspond to transition states of low dimensionality less than 2. The agreement of the lasing wavelengths with the δ-like absorption peaks suggests that the transition states are confined in quantum dots and/or quantum wires formed by the strain-field profile and the InGaAlAs alloy

segregation/migration within the QWs. Moreover, lasing operation at excited transitions of high modal gain have been reported for quantum-dots and quantum-wires lasers [4.7], [4.8].

The laser performances are consistent with the quantum dots/wires lasers. The energy separation for the lasing peaks is about 7-9 meV, which is also consistent with the calculated hole level separation for InGaAs quantum dots [4.7]. For the quantum structure of n-type modulation doping, the lasing peaks are controlled by the level occupation of excess holes.

The excess injected holes thermally relax to the higher level in the quantum dots. Since within the quantum dots, hole thermal relaxation process of the excited state is dominated by inelastic multi-phonon scattering, and presents a relaxation bottleneck to populate the underlying states [4.9], [4.10]. The relaxation time takes a few of 10 ps [4.10], which is longer than the stimulated radiative recombination time. At injection current above threshold condition, lasing transition at high energy state is observed. As the injection current increased, more excess holes relax to the next lower level, and the second lasing peak appears in addition to the main peak. Further increasing the injection current, the process goes on as long as there exists underlying hole levels. For our case, we have observed three peaks at I = 130 mA for the FP laser. From the photocurrent spectrum in Fig.

4.10, we would expect that the fourth lasing peak at λ = 1560 nm (7.7 meV separation to peak at λ = 1545 nm) will expose at higher injection current. However, the wavelength-dependent modal gain distribution of the diode laser can hinder the process as shown in Fig. 4.9 for the TEF-laser operating at the excited transition λ = 1511 nm.

Fig. 4.6. (a) PL spectra, and (b) EL spectra for sample “MD3QW” at room temperature.

Fig. 4.7. The characteristics of current density vs. applied forward biases (J-V).

(Website of nextnano³: http://www.wsi.tum.de/nextnano3/index.htm)

Fig. 4.8. FP-type ridge laser: (a) L-I characteristics, and (b) lasing spectra.

Fig. 4.9. TEF-type ridge laser: (a) L-I characteristics, and (b) lasing spectra.

Fig. 4.10. Photocurrent spectrum measured at V = 0.

Reference

[4.1] D. Marcuse, “Reflection loss of laser mode from tilted end mirror,” J. Lightw.

Technol., vol. 7, pp. 336-339, 1989.

[4.2] H. P. Fan, “Photocurrent and Electroabsorption spectroscopy for Semiconductor Quantum Well structures,” Master thesis, Inst. of Electro-optical Engineering, SYSU, 2001.

[4.3] A. Niwa, T. Ohtoshi, K. Uomi, and K. Nakahara, “Doping-type dependence of turn-on delay time in 1.3　m GaAsP-InP modulation-doped strained quantum-well lasers,” IEEE Photon. Technol. Lett., vol. 8, pp. 328- 330, 1996.

[4.4] V. D. Kulakovskii, E. Lach, and A. Forchel, “Band-gap renormalization and band-filling effects in a homogeneous electron-hole plasma in In0.53Ga0.47As/InP single quantum wells,” Phys. Rev. B, vol. 40, pp. 8087-8090, 1989.

[4.5] P. Bhattacharya, Semiconductor Optoelectronic Devices (2nd, Prentice-Hall, New Jersey, 1994), pp.175.

[4.6] (Website of nextnano3: http://www.wsi.tum.de/nextnano3/index.htm)

[4.7] G. Park, O. B. Shchekin, and D. G. Deppe, “Temperature dependence of gain saturation in multilevel quantum dotlasers,” IEEE J. Quantum Electron., vol. 36, pp.

1065-1071, 2000.

[4.8] E. Kapon, D.M. Hwang and R. Bhat, “Stimulated emission in semiconductor quantum wire heterostructures,” Phys. Rev. Lett. vol. 63, pp. 430-433, 1989.

[4.9] U. Bockelmann, and G. Bastard, “Phonon scattering and energy relaxation in two-, one-, and zero-dimensional electron gases,” Phys. Rev. B, vol. 42, pp. 8947-8951, 1990.

[4.10] R. Heitz, M. Veit, N. N. Ledentsov, A. Hoffman, D. Bimberg, V. M. Ustinov, P. S.

Kop’ev, and Zh. I. Alferov, “Energy relaxation by multiphonon processes in InAs/GaAs quantum dots,” Phys. Rev. B, vol. 56, pp. 10435-10445, 1997.

Chapter 5

Electro-Absorption Characteristics in N-type Modulation-Doped Strain-balanced MQWs

Because the QWI process is not really presented to blue shift our λ = 1.55 μm samples in this study, six additional 1.48 μm samples have been designed and grown to simulate a 70-nm blue-shifted material without actually carrying out the QWI process. In fact, these samples with different n-type modulation-doped (MD) distribution are designed with structure quite similar to that of 1.55 μm one but with reduced layer thickness inside the well to shift the transition wavelength (e1-hh1) to 1.48 μm. These wafers are used to study the relations between Δn, differential absorption (Δα), and MD distribution.

5.1 Six blue-shifted samples (λ = 1.48 μm)

According to the design concept of MD3QW, one period of the blue-shifted triple-QWs as indicated in Fig. 5.1 can achieve a high value of wavefunction OIS (e1-hh1) of 0.95; simultaneously, it is optimized by a thin 1.6-nm lattice-matched InGaAs as core layer, a pair of 1.1-nm compressive-strained InGaAs as padding layer, a pair of wavefunction-OIS-enhancement layers formed by 1.5-nm compressively-strained InAlAs wavefunction-adjustment layers and thin 1.2-nm tensile-strained InGaAlAs spacer layers, and totally strain balanced by a twin of 3.5-nm most outside tensile-strained InGaAlAs barrier. Hence, the separation between any two QW’s is 7.0-nm. Six different λ = 1.48 μm samples having different doping distribution with/without hole-stopping layer are prepared for investigating their differential absorption properties.

Besides sample 1 (S1) has two periods of QW’s, the other five samples (S2-S6) have

three periods of QW’s. A portion of tensile-strained InGaAlAs barrier is Si-doped to 1 × 10¹⁸ cm^-3 to produce n-type modulation-doping. S1 and S2 have modulation-doping covering full thickness of barriers amounting to a sheet density of 7 × 10¹¹ cm^-2 per QW; S3 and S4 have modulation-doping covering inside half thickness of barriers amounting to a sheet density of 3.5 × 10¹¹ cm^-2 per QW; S5 is only covering quarter thickness of barriers amounting to a sheet density of 1.75 × 10¹¹ cm^-2 per QW, but MD is not involved inside the barrier of S6. On the other hand, a hole-stopping barrier near to n-side of p-i-n structure is further introduced to S4, S5, and S6 as demonstrated in Fig. 5.1 (indicated by arrow with dot-line). The differences between these six samples are also summarized in Table 5-1.

These samples are prepared to study the relation between refractive index change (Δn), differential absorption (Δα), and modulation doping distribution.

Fig. 5.1. Band diagram and subband positions of e1 and hh1 for blue-shifted samples.

Table 5-1 Six blue-shift samples.

Samples Number of QW MD Thickness @ Barrier

(Which inside parenthesis ( ) is corresponding to the serial index of sample in our group.)

5.2 Photocurrent and electroluminescence (EL) spectra

In order to investigate the MD distribution and hole-stopping layer how to influence electro-absorption properties between the blue-shifted samples, in first, these samples are processed as mesa diodes for measuring photocurrent, EL, and Δα spectra.

For later discussion, Figures of 5.2 – 5.7 sequentially show the room-temperature photocurrent spectra at reversed bias and EL spectra at forward bias for six different samples (S1 – S6). Depending on those photocurrent spectra, a phenomenon which photocurrent signals near bandedge from modulation-doping samples having a relative higher absorption intensity with a little of red shift as the applied reversed voltage increased, has been obviously observed. Moreover, in case of samples with higher sheet doping density, this phenomenon is more outstanding than those with lower sheet doping density. On the other hand, which absorption intensity increasing as the applied reversed voltage increased is not frequently happened; it is also against to the quantum-confined-Stark-effect (QCSE).

Furthermore, the other two samples, S5 having a sheet density of 1.75 × 10¹¹ cm^-2 per QW and S6 without modulation doing, do not perform any considerable change in photocurrent spectra near bandedge, it simultaneously means that the QCSE can be almost neglected in our compact QW design. In other words, how much carriers depleted inside barriers will

dominate this effect. Further, a noticeable exciton peak in photocurrent spectra with an abrupt absorption edge from the sample without modulation doping of S6 has demonstrated that our MBE samples have good enough epitaxial quality.

Although the water absorption peaks existing in the measurement system cause the shape of EL emission spectra to have a lot of depression in the wavelength range of 1.35-1.4 μm, it is still obvious the higher excited wavelength emission will be enhanced as the injecting current increased sequentially. Three vertical arrows assist us in pointing out those calculated transition energy of samples in the wavelengths of 1.48 μm (e1-hh1), 1.38 μm (e1-hh2), and 1.33 μm (e1-hh3) are corresponding to those transition peaks presented in EL spectra and approximately match to the photocurrent results.

To compare the FWHM of EL spectrum operating under 70mA, the results show S1 and S2 with MD of 7 × 10¹¹ cm^-2 per QW are found to have more extensive FWHM of 132nm and 142nm; S3, S4, and S5 with MD in range of 3.5 ~ 1.75 × 10¹¹ cm^-2 per QW are found to have middle FWHM in range of 114 ~ 120nm; however, S6 without MD is found to have the narrowest one of 102nm.

Depending on the results of photocurrent and EL spectra, in short summary, we realize:

(1) the compact blue-shift QW doesn’t present obvious QCSE under strong reversed bias, (2) the absorption intensity of MD samples is tunable and samples having higher MD sheet density perform larger tunable range, (3) these blue-shift samples exist three TE-polarized energy transition; also corresponding to our simulation results, (4) higher MD density cause larger FWHM in EL emission spectra, (5) hole-stopping layer doesn’t matter in photocurrent and EL measurement results very much.

5.3 Transmission and Absorption

Because it is not easy to obtain the Δα spectroscope [5.1] directly, the change of transmission (ΔT/T) in the presence of electric field is used to study the electroabsorption (EA) modulation behavior of our blue-shift MD-SOA’s samples. As the source light is perpendicularly inserted into a uniform semiconductor material, the light intensity will be exponentially decayed as its penetrating thickness increased. Thus, the transmission can be expressed as

T I

I e ^α

(5.1)

where dcore is regarded as the total active region thickness of epi-layer and α is the absorption coefficiency. To introduce a reversed bias into the transmission measurement, the change of transmission in case of (V – 0) modulating biases can be expressed as

ΔT V 0

Therefore, the differential absorption (Δα) in condition of modulated biases from 0 volt to the reversed one can be deduced from Eq. (5.2) as given below

Δα V 0 ln 1 ΔT V 0 T 0 d

(5.3)

The modulated refractive index change (Δn) is then obtained from the Δα spectra through Kramers-Kronig Transform (KKT) [5.2], [5.3].

5.3.1 Kramers-Kronig Transform (KKT)

According to the KKT, we know the relation between Δα and Δn can be shown as below In application of Eq. (5.4), however, it is not possible to have an unlimited experimental data for this integral. In addition, this integral formula also exist divergent point. In order to prevent this hassle, we simplify Eq. (5.4) as the following alternative

Δn_V λ λ However, using this alternative of Eq. (5.5) must to match the condition of λ^′ λ λ λ^′ .

5.3.2 Experimental results and discussions

A combination of a tungsten lamp and a monochromator is used as the light source.

The transmission spectrum (T) is measured in a surface normal configuration of mesa diode.

The ΔT spectrum is measured in the same configuration with the diode reverse biased by a square-wave voltage. The lock-in technology is applied to filter signal noises between Ge detector and voltage wavefunction generator. In order to ensure a good accuracy for the Δn spectra by KKT, the ΔT/T spectra are taken over a wide range of λ from 1.0 to 1.7-μm to include all the significant spectral features.

By this method, the Δn and Δα spectra for six blue-shift MD-SOAs samples have been investigated as shown in Figures of 5.8 – 5.13. Each spectrum line in the figures is corresponding to which sample is modulated by bias between 0 volt and the reversed ones indicated in the legend. As mentioned above, which Δα spectra are deduced from ΔT/T spectra, referring to Eq. (5.3). Using KKT method, Δn spectra are further obtained by Δα spectra, referring to Eq. (5.5). Therefore, which values in Δn and Δα spectra are corresponding to the changes of refractive index and absorption coefficient from 0 volt to the reversed ones; in other word, it means Δα = α(V) - α(0) and Δn = n(V) - n(0).

In Fig. 5.14, those chirp parameters, Δn/Δk, where Δk = λΔα/4π, for six blue-shift samples extracted at λ = 1.55 μm, are plotted as functions of the reversed bias modulation amplitude. A high value of chirp parameter is also desirable for electro-refractive devices that require pure optical phase modulation with very little intensity modulation caused by electro-absorption. As we see it, the Δn/Δk is decreased with the applied voltage increasing.

In detail, (1) S6 without MD has the smallest value of Δn/Δk; (2) among those two samples having same MD covering in full thickness of barrier but different period of QWs, S2 including three periods of QWs has a higher value of Δn/Δk than S1 including only two periods of QWs; (3) among those two samples having three periods of QWs but different MD distribution, S3 with MD in half thickness of barrier has a higher value of Δn/Δk than S2 with MD in full thickness of barrier; (4) in comparison of both samples having three periods of QWs and MD covering in half thickness of barrier but either is with hole-stopping barrier, it is found that S4 which hole-stopping barrier is involved has a larger value of Δn/Δk than S3; (5) also, S4 has a lot of improvement in value of Δn/Δk than S5, which structure is as well as S4 but barriers are only quarter thick covered by MD. To compare the value of Δn/Δk between these samples, it is found for S4 > S5 > S3 > S2 > S1

> S6; simultaneously, Fig. 5.11 for S4 obviously indicates that the positive Δn spectrum peak occurs at the tail of Δα spectrum near λ = 1.55 μm. In summary, among these six

samples, S4 with modulation-doping at sheet density of 3.5 × 10¹¹ cm^-2 per QW and with a hole-stopping barrier represents the best design for electro-refractive applications. Therefore, the structure of S4 offers an excellent platform to realize electro-refractive devices with larger Δn but lower Δα near 1.55 μm which is our interested region of operation.

Fig. 5.2. Sample 1: (a) photocurrent spectra and (b) EL spectra at T = 300K.

Fig. 5.3. Sample 2: (a) photocurrent spectra and (b) EL spectra at T = 300K.

Fig. 5.4. Sample 3: (a) photocurrent spectra, and (b) EL spectra at T = 300K.

Fig. 5.5. Sample 4: (a) photocurrent spectra and (b) EL spectra at T = 300K.

Fig. 5.6. Sample 5: (a) photocurrent spectra and (b) EL spectra at T = 300K.

Fig. 5.7. Sample 6: (a) photocurrent spectra and (b) EL spectra at T = 300K.

Fig. 5.8. Sample 1: (a) Δn spectra and (b) Δα spectra at T = 300K.

Fig. 5.9. Sample 2: (a) Δn spectra and (b) Δα spectra at T = 300(.

Fig. 5.10. Sample 3: (a) Δn spectra and (b) Δα spectra at T = 300K.

Fig. 5.11. Sample 4: (a) Δn spectra and (b) Δα spectra at T = 300K.

Fig. 5.12. Sample 5: (a) Δn spectra and (b) Δα spectra at T = 300K.

Fig. 5.13. Sample 6: (a) Δn spectra and (b) Δα spectra at T = 300K.