Spontaneous polarization effects on the optical properties of piezo-strained InGaN quantum wells

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carrier with a different frequency by adjusting the DC bias, this is particularly useful in wireless systems that use frequency diversity to reduce cross talk among adjacent cells. The output of each laser is split into M carriers hence a single self-pulsating DFB is shared among M cells using the same carrier frequency. Each carrier is modu- lated then directed to a different multiplexer. Each multiplexer combines carriers with differ- ent wavelengths and transmits them through a single fiber, therefore N wavelengths share the same fiber. Another advantage of WDM is it lim- its the speed of the electronics needed at the base station to the bit rate of the individual cell. If the cells are scattered and cross talk is of no impor- tance, WDM allows each cell to use the full RF bandwidth. The figure demonstrates how a clus- ter of seven cells are able to share the fiber from the control station then use a demux to supply a different millimeter-wave carrier to each cell. The output of each multiplexer may be configured according to need.

The basic principles of the network is demon- strated using two 60 GHz self-pulsating DFB lasers emitting at 1548.4 nm, and 1549.9 nm. First the output of the lasers are split, then a mul- tiplexer is used to combine carriers of different wavelengths into the same fiber, enabling the support of four cells over two fibers. The WDM/self-pulsation signal is transmitted over 14 km of fiber, at the receiver end a filter is used to extract the designated self-pulsation. The opti- cal spectra of the transmitted WDM/self-pulsa-

tion and the filtered self-pulsation at the receiver are shown in figure 2. For data transmission the lasers are sub-harmonically injection locked then modulated with a 155 Mb/s PSK data at a 1 GHz offset. The RF spectrum of a modulated 60 GHz self-pulsation is in figure 3 along with the phase noise of the carrier. The spectrum is measured by optically downconverting the received signal by 17 GHz. The figure compares the actual mea- sured phase noise with the estimated phase noise when the penalty due to optical downconversion is taken into consideration.

A network architecture based on wave divi- sion multiplexing of selfpulsating DFB lasers for 60 GHz wireless systems is proposed and demon- strated. The network offers sharing of resources and combines the advantages of remote genera- tion of millimeter-wave carrier and WDM.

References

1. G. Smith, D. Novak, and Z Ahmed, “Over- coming Chromatic-Dispersion Effects in Fiber-Wireless Systems incorporating Exter- nal Modulators,” IEEE Trans. Microwave Theory Tech., Vol. 45, No. 8, August 1997, p.1410-1415.

M. AL-mumin, X. Wang, W. Mao, S. Paper, and G. Li, “Optical generation and Sideband Injection Locking of Tunable 11-120GHz Microwave/Millimeter Signals”, Elect. Lett. vol. 36, N0.18,August 2000, pp 1548-1549. M. AL-mumin, X. Wang, W. Mao, G. Li,“Op- tical Generation and Self Sub-harmonic In-

2. 3.

1546

1548

1550

1552

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1546

1548

1550

1552

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CThL56 Fig. 2.

consists of a grating and a circulator). Right-The filtered output at the receiver.

Left-Multiplexing of the two self-pulsations at the transmitter (The multiplexer

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CThL56 Fig. 3. (a) 155 Mbls PSK signal transmitted on a 60 GHz carrier, (b) Phase noise of the 60 GHz carrier, solid-actual measured phase noise, dash-phase noise when the penlaty of optical down- conversion is considered.

jection Locking of Tunable 10-100 GHz Mi- crowavelMillimeter Signals”, in Conference on Lasers and Electro-optics, OSA Technical Digest, May 2000, pp. 96-97.

CThL57 1 0 0 pm

Spontaneous polarizatlon effects on the optical propertles of piezestrained InGaN quantum wells

L.-H.Peng,K.-T. Hsu,C.-W.Shih,C.-C.Chuo,* J.-I. Chyi,* Institute of Electro-Optical

Engineering, National Taiwan Universiv, Taipei,

106 Taiwan, R.O.C.; email: peng

@cc.ee.ntu.edu.tw; *Department of Electrical

Engineering, National Central University, Chungli, 320 Taiwan, R.0.C; email: [email protected]. tw

Rapid development in the epitaxial growth, dop- ing control, and device processing on group I11 nitride has brought in a plethora of research ac- tivity. Although much attention has been empha- sized on the device application, ambiguities still exist on the emission mechanism of InGaN quantum wells (QWs). Due to the inhomogene- ity issues in the material growth, it has been sug- gested the efficient radiative recombination of InGaN QW is due to the localized carriers at the band-tail states. Others suggest the emission is from the highly localized, quantum dot-like states in the phase-separated In-rich regions in the well. Controversy also remains on the gain and lasing mechanism of the nitride lasers. While the more traditional argument favors the mecha- nism of electron-hole plasma recombination originating from free carriers, some suggest the carrier localization in the plane of the QW layers can enhance the quantum efficiency.’

In resolving the fundamental emission mech- anism of 111-V nitride, it is recently noticed that the discontinuity of macroscopic polarization can induce 2D-electron or hole gas at the inter- face.2 The polarization-induced charge is related to the piezoelectric- and the spontaneous-polar- ization of the wurtzite structure. In GaNlAlGaN QWs, the difference in the spontaneous polariza- tion can result in a strong field in the well even with the absence of the piezoelectric field.3 Com- bined with the information shown above, it is de- sirable to design an optical study immune from the localization effects such that contribution from the spontaneous polarization effects can be clearly resolved.

In this work, high-excitation spectroscopy is used to study the optical properties of strained InGaN QWs. By engineering the spontaneous polarization induced charge to have opposite sign in the symmetric and asymmetric QWs, large spectral blue shifting (- 140 meV) and linewidth narrowing (- 10 meV) are observed. These effects are attributed to the charge screen- ing of the polarization field and enhanced radia- tive recombination in the electron-hole plasma regime.

The 3.0 nm Ino.15 Ga.,.ssN QW samples used in this study were grown on a -1.5 km GaN buffer layer on the (0001) sapphire substrate in an AIX 200/4 low-pressure MOCVD ~ y s t e m . ~ In order to investigate the polarization effects, symmetric GaNIInGaNIGaN and asymmetric GaN/In- GaN/AlGaN QW structures were used with a top

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THURSDAY AFTERNOON barrier of 50 nm thickness. In doing so, the

piezoelectric field in the compressive-strained InGaN QW is to point toward the substrate di- rection. Moreover, the design of the symmetric InGaN QW is to have the spontaneous polariza- tion induced charge opposite to that of the asym- metric QW case. As a result, the piezo- and the spontaneous- polarization effects add up in the latter case whereas they temp to compensate each other in the formal case. To ensure a complete filling of the localized states, the optical study was made with a KrF excimer laser excitation (Tui- Laser ExciStar 2000) with a pulse width of 10 ns, repetition rate of IO Hz, and maximum pulse en- ergy of 14 mJ. The experiments were taken in a surface emission configuration to minimize the reabsorption effect on the photo-luminescence

(PL) spectra. The data were recorded by a spec- trometer equipped with a CCD array detector.

Shown in Fig. 1 (a) are the normalized room- temperature time-integrated room-temperature PL spectra of the 3.0 nm Ino lsGa,,85N QWs, and (b) the corresponding emission linewidth de- pendence on the instant excitation power densi- ty. We first note a large

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140 meV spectral shift- ing between the 3.0 nm InGaN QWs capped with different top barriers. Moreover, with the in- crease of the excitation density, the the high-en- ergy part of the spectra is found to take an expo- nential dependence on the energy. This feature signifies the hot-carrier e f f e ~ t . ~ We also note a pronounced linewidth narrowing effect with the pump density. Using this pump technique and material choice, linewidth as narrow as 10 meV can be achieved in the symmetric InGaN QW at room temperature.

Shown in Fig. 2 are the self-consistent analysis on the emission spectra of the 3.0 nm InGaN QWs taken into account the many-body, charge screening, piezoelectric- and spontaneous-polar- ization effects6 A carrier injection of N , , = 1.5 x l O I 9 is assumed in the analysis. It is found the compensation mechanism of the sponta- neous- and piezoelectric-polarization in the symmetric InGaN QW can greatly reduce the in- ternal field in the well and lead to a substantial spectral blue shifting and enhancement in the emission intensity. In the regime where the linewidth narrowing takes place, the calculation indicates the transition is mainly due to the free carrier recombination in the QW subbands. Fur- ther shown in Fig. 3 are the high-excitation PL spectra of the 3.0 nm symmetric GaN/InGaN/ GaN QW at a pump intensity greater than 10

MW/cm2. The salient features in the transition from the linewidth broadened (-20 meV) to the multi-peak emission spectra clearly reveal the subband-filling effects. These observations sug- gest that in the high-excitation regime, the mech-

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CThL57 Fig. 2. Calculated optical emission spectra of the symmetric and asymmetric 3.0 nm InGaN QWs.

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CThL57 Fig. 3. High-excitation PL spectra of symmetric 3.0 nm GaN/Inn.,5G%.85N/GaN QW.

anism of electron-hole plasma recombination is responsible for the InGaN QW emission spectra. In summary, we show the engineering of spontaneous polarization in the symmetric In- GaN QWs can enhance the optical transition en- ergies and emission intensity. In the high-excita- tion regime, the mechanism of electron-hole plasma recombination is found to dominate in the emission spectra. This research was spon- sored by the NSC Grant No. 89-2215-E-002-041 and 047.

1. Y.-H. Cho, T.J. Scmidt, S. Bidnyk, G.H. Gain- er, J.J. Song, S. Keller, U.K.. Mishra, and S.P. DenBarrs, “Linear and nonlinear optical properties of InGaN/GaN heterostructures,” Phys. Rev. B 61,7571-7588 (2000).

V. Fiorentini, E Bernardini, ED. Sala, A.D. Carlo, and P. Lugli, “Effects of macroscopic polarization in 111-V nitride multiple quan- tum wells,” Phys. Rev. B 60, 8849-8858 (1999).

3. S.-H. Park and S.-L. Chuang, “Spontaneous polarization effects in wurtzite GaNlAlGaN quantum wells and comparison with experi- ment,” Appl. Phys. Lett. 76, 1981-1983 (2000).

C.-C. Chuo, C.-M. Lee, T.-E. Nee, and J.-I. 2.

4.

, - .

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.

CThL57 Fig. 1.

= 1 MW/cm*, and J, = 2.4 MW/cm2, (b) linewidth dependence on the excitation power density. (a) normalized PL spectra of the symmetric and asymmetric 3.0 nm InGaN QWs. Jn

Chyi, “Effects of thermal annealing on the luminescence and structural properties of high indium-content InGaN/GaN quantum wells,” Appl. Phys. Lett. 76, 3902-3904

EBinet, J.Y. Duboz, J. Off, and E Scholz, “High-excitation photoluminescence in GaN: hot-carrier effects and the Mott transi- tion,” Phys. Rev. B 6 0,471 547 22 (1999).

L.-H. Peng, C.-W. Chuang, and L.-H. Lou, “Piezoelectric effects in the optical proper- ties of strained InGaN quantum wells,” Appl. Phys. Lett. 74, 795-797 (1999).

(2000). 5.

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CThL58 1 0 0 pm

Stability properties of disperslve extendedaavity semiconductor lasers

L. Ramunno, J.E. Sipe, Department ofphysics,

University of Toronto, 60 St. George Street, Toronto, Ontario, Canada, M5S lA7

Since semiconductor diode dispersive extended- cavity lasers are currently of interest for a variety of applications,”’ understanding their stability properties is important to ensure stable CW laser operation. These lasers consist of a semiconduc- tor diode coupled to some sort of external dis- persive reflector (e.g. a fiber grating) that forms one mirror of the laser cavity; see Fig.1 for a schematic. Quite unexpectedly, experiments have shown that for chirped fiber grating lasers (FGL), the orientation of the gratin drastically alters the stability of CW operation: when the grating was placed such that the index modulation peri- od decreased with distance from the coupled diode facet, stable single mode operation oc- curred, but the opposite grating orientation re- sulted in significant mode-hopping. This result is counter-intuitive, since the grating reflectivity spectra are identical in both cases; the only dif- ference is in the sign of the curvature of the re- flection spectrum phases, and this has been shown to play only a small role in large-scale cur- rent modulation

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In this presentation, we not only explain these curious experimental results, but we also find a simple numerical prescription with which the stability of any such system can be assessed, pro- vided the reflection spectrum of the external re- flector is known. We perform a linear stability analysis wherein we seek the response of the de- viation of the electric field from steady state,

w

( t ) , given a small perturbation from the steady

state. Using an accurate but simple model for the semiconductor diode developed earlier: we find an expression for the Laplace transform of y~ ( t ) ,

denoted by $(s), that is given explicitly in terms of the reflection spectrum of the dispersive re- flector as a function complex frequency; this spectrum is easily calculated for a fiber grating, for example. We then determine laserA stability simply by locating the singularities of ~ ( s ) over

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dispersive (titer grating) reflector

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semiconductor diode

CThL58 Fig. 1. Schematic drawing of a semi- conductor diode dispersive extended-cavity laser.