Conclusion

Ion channeling technique using MeV C⁺⁺ ions was used to study strain in self-assembled InAs QDs buried in GaAs matrix. Because of the use of heaving ions, we were able to observe an angular shift in the angular scan of the In signal relative to that of the Ga/As signal. This provided a direct evidence that the InAs lattice is larger than that of GaAs in the growth direction. Combing the channeling results in [100] and [110] directions and the photoluminescence emission spectrum, we conclude that the InAs QDs are under tensile strain in the growth direction and have the same lattice constant as that of GaAs in the lateral direction. Thermal annealing causes the strain to relax, first in the growth direction and then

in the lateral direction as the annealing temperature increases. The photoluminescence spectra of the QDs before and after annealing indicate, however, that composition intermixing also takes place during annealing and is the dominant factor in determining the band gap energy of the QDs.

Chapter 4

The wavelength switching Transition in quantum dots lasers

4.1 Introduction

InAs self-assembled quantum dots (QDs) generally exhibit two distinct peaks in photoluminescence spectra corresponding to the ground and the excited state transitions. It is of great interest that whether these two transitions can have properties of the two distinct state lasing, which may be one important development in the laser research. The possibility of two-state lasing had been demonstrated in theory by Grundmann et al [45-46]. Simultaneous lasing at two well-separated wavelengths has been experimentally shown in self-assembled InAs QD lasers via ground state and excited state transitions [47-49]. However, the possibility of lasing switching between two wavelengths has not been completely studied. In this chapter, we present a detailed lasing switching behavior between the ground state and the excited state in InAs QD lasers. The lasing switching behavior is presented in two parts: (1) convention single cavity laser; (2) couple cavity laser system.

4.2 Device Growth

The QD lasers were grown on (100)-n+ GaAs substrates using molecular beam epitaxy (MBE). The epi-layer structures of the lasers consisted of, starting from the bottom, a 1.2 μm

1.2 mm n-Cladding Al_0.6Ga_0.4As

FIG. 4.1. The growth sequence of laser is schematically shown. The hetero-structure for QDs were grown by MBE.

thick n-type Al0.4Ga0.6As cladding layer with 3x10¹⁷ cm^-3 Si doping, a 0.4 μm thick undoped GaAs waveguide layer, a 1.2 μm-thick p-type Al0.4Ga0.6As upper cladding layer with 3x10¹⁷ cm^-3 Be doping, and finally a 200 nm-thick p⁺ GaAs contact layer. The active region consists of three InAs QD layers, which reside in the middle of the waveguide layer. Each QD layer is capped by a 6 nm In0.15Ga0.85As layer. A 40-nm GaAs barrier layer is inserted between the QD layers. In growth rate for QDs and capping layer is 0.056 μm/hr. The layer structure of the laser is schematically shown in Fig. 4.1.

4.3 Device Process

Conventional ridge waveguide structure was used for QDs laser fabrication. The cavity width is 20 μm. The cavity lengths are varied from 1000 to 1500 μm. Typically, the laser devices studied in this dissertation were processed with a standard procedure. To confine optical field in lateral direction, shallow mesa was necessary. First, the desired device patterns were processed by standard photo-lithography steps. That is, photo-resist coating, soft baking, exposure under UV light, development in developer solution, rinse in DI water and dry by pure nitrogen purge. After photo-lithography steps, p-type ohmic contacts of Ti/Pt/Au were deposited. All metal contacts were formed by E-gun evaporation, the chamber pressure was lower than 2x10^-6torr. Before metal formation, the surface was treated with the following steps. First, to remove residue photo-resist, the surface was treated with UV/O3 stripper at temperature of 100^oC for 2 minutes. The native oxide on the surface was etched by using the solution of HCl: H2O (1:1) for 30 seconds. After native oxide removal, the wafers were rinsed in DI water and then dried by pure nitrogen. In order to prevent from re-oxidation of the surface in the air, the wafer was loaded into the E-gun chamber quickly after surface treatment. When the wafers were unloaded from E-gun evaporation, Lift-off in acetone (ACE) could made the p-contacts strip patterns appear.

The shallow mesa of the ridge lasers were formed by using wet chemical etching. In details, solution of H2SO4: H2O2: H2O (1:8:40) with an etching rate of about 20 nm/s was used for mesa formation. All etching process was carried out at room temperature. For all conventional ridge waveguide structure, whether the lasers were single or couple cavity, etching stop in the interface between p-cladding and GaAs waveguide.

The metal alloy in n-ohmic contacts were different from the p-ohmic contacts, therefore, n-contacts were formed after the shallow mesa formation. To cleave lasers cavity with different length, the wafers should remove their thickness on substrate side before depositing

n-metal alloy. Generally, the 200 μm thick n-substrate should be removed with chemical wet-etching solution (NH3OH:H2O2=1:3). Ni/Ge/Au n-contacts were formed by E-gun evaporation; the chamber pressure was also lower than 2x10^-6torr. Before n-metal formation, the surface was also treated for removing native oxide. The steps had been expressed above.

The rapid thermal annealing (RTA) step was performed after n-metal contact formation. The n-metal contact becomes ohmic contact after the RTA step. RTA temperature is 400^oC for 30 second. Figure 4.2 shows the fabrication procedure of our lasers.

P-contact pattern

FIG.4.2. The conventional ridge waveguide structure was fabricated with standard procedure.

4.4 Characteristic Measurement System

Laser light- current (L-I) measurements were performed under pulsed condition. The pulsed currents were generated by as HP8114A pulsed generator. The light output is detected by a calibrated Si or InGaAs PD with 9 volts reverse-biased. The injection current and the detection current are then gated and integrated by the Standford research system SR250 gated integrator and boxcar averager. In our experiment, the pulse width was 1 μs and the duty cycle was 5%. Fig. 4.3 shows the schematic of the L-I measurement system. The spectra of the quantum dot lasers were measured by an optical analyzer. The lasers were characterized in the temperature range of 20-60 ^oC.

FIG 4.3. The schematic diagram for the measurement of L-I characteristics

4.5 Lasing switching in single cavity QD lasers

4.5.1 Using Different Drive Currents

Fig. 4.4 (a) and (b) show the output power and lasing spectra of an as-cleaved OD laser at different drive currents. The cavity width and length were 20 μm and 1400 μm, respectively. The laser was operated at 20°C. In Fig.4.4 (a), the L-I curve reveals that the ground state threshold is 44 mA. With increasing injected current, we observe that the slope of L-I curve changes at certain point. The lasing spectra at different driving currents have also been measured. The results are shown in Fig 4.4(b). The ground state transition appears around 1.23 μm at 44 mA. The output power of the ground state emission saturated with bias current at 120mA.With increased drive current to around 130mA, an additional spectral band appears around 1.124 μm, which corresponds to the wavelength of the first excited state transition. Between 130 mA and 190 mA, both the ground state band and the excited state band exist together. By comparing with the results in Fig.4.4(a), the change of the slope in L-I curve occurs at the point that the output power of the ground state emission saturated and the excited state lasing starts to take place. When the higher current I s used, the output power of the ground state lasing gradually disappears. Finally, the lasing completely switches from the ground state transition to the excited state transition.

In conclusion, the lasing wavelength of QD lasers depends on the driving current. The ground state lases first, followed by the excited state. Then, two-state lasing occurs. Finally, lasing wavelength switches to the excited state transition. The change of slope in L-I curve indicates that the ground state power saturated and the excited state lasing threshold reaches.

(a)

(b)

FIG. 4.4. The results L-I curve of QDs laser and the optical spectrum with varied bias currents, respectively. In the QDs laser, cavity width is 20 μm and cavity length is 1.4 mm.

4.5.2 Cavity Length and Operation Temperature

The L-I curves of QD lasers with various cavity lengths and operation temperatures are depicted in Fig. 4.5(a)-(e), where show mode switching for the cavity length varied from 1100 (Fig. 4.5(a)) to 1500 μm (Fig. 4.5(e)) with a step 100 μm and at various temperatures (20-40°C). Optical output power spectra are also monitored simultaneously, but the results do not show here. Fig. 4.5(a) shows that lasing threshold occurs at 80mA. Combined with the results of optical output spectra, we find that the laser only lases at the excited state transition.

At such cavity length (i.e. 1100μm), the gain from the ground state transition is too small to compensate for the total loss. Fig. 4.5(b) represents the L-I curves of the 1200 μm long laser.

At 20°C, a lasing threshold occurs at lower current than that in 1100 μm length laser (Fig.

4.5(a)). The slop of L-I curve changed at higher driving currents. The corresponding optical output power spectrum indicates that the lasing starts with the ground state transition. The excited state lasing starts at where the slope of L-I curve changes. When the temperature is raised to 25°C, only excited state lasing is observed.

Three L-I curves with three different temperatures (20°C, 25°C, 30°C) and the same laser cavity length (1300 μm) are shown in Fig.4.5(c). The ground state threshold occurs at 20 and 25°C operation temperature. At 30°C operation temperature, lasing changes from the ground state transition to the excited state transition. The wavelength switching between these two transitions occur at higher temperature. The change of the slope appears at lower current as the operation temperature increased. It indicates that the excited state threshold decreases with higher operation temperature. Fig. 4.5(d) and (e) show results at five different temperatures for cavity length at 1400 and 1500 μm, respectively. These results are similar to those in Fig. 4.5(c). The ground state (excited state) threshold increases (decreases) with increasing the operation temperature. When the temperature is higher than a certain temperature, only excited state lasing is observed. This certain temperature increases when

cavity length increases. For the 1500 μm long laser, the excited state threshold did not occur at high injected current neither at a temperature range of 20 to 30°C. The excited state lasing threshold occurs when the temperature was higher than 30°C.

From above results, we found that two-state lasing occurs at a laser cavity of 1200 to 1500 μm and at room temperature. The shorter devices only lase at the excited state transition.

The ground state transition only occurred at longer devices. The range of the cavity length that two-state lasing would occur varied with different operation temperatures.

(a) 1100 μm

(b) 1200 μm

(c) 1300 μm

(d) 1400 μm

(e) 1500 μm

FIG. 4.5. L-I curve of QDs laser with various cavity lengths length (a)1.1mm (b)1.2mm(c)1.3mm(d)1.4mm(e)1.5mm. L-I curves were measured at various operation temperatures.

The basic parameters of QDs lasers, internal quantum efficiency (ηi) and internal loss could be determined from L-I curves with various cavities. The external differential (ηd) quantum efficiency is proportional to the slope efficiency (dL/dI) and is defined as follow:

( )

where R is the power reflectivity of at the mirror of cavity. For GaAs semiconductor laser, R is 0.32 for as-cleave mirror.

The reciprocal of the external differential quantum efficiency can expressed as follow:

^{η1d ≡η1i ⎜⎜}_⎝^⎛1+lnα

( )

L is the cavity length. From the equation, internal loss could be obtained when a laser cavity had different cavity length. The internal quantum efficiency could be determined from (4.3).

Fig. 4.6 shows the inverse of the slope efficiency as a function of cavity length in our QD lasers. Using equation (4.1) and (4.3), the internal loss is 4 cm^-1 and internal quantum efficiency is 98%.

FIG. 4.6. The inverse of the slope efficiency is the function of cavity length.

4.5.3 Conclusion

The wavelength switching behavior (from the ground state to the excited state transition) of InAs QD lasers was investigated in this section. The results show that wavelength switching could be achieved by driving the device at different currents, and changing the cavity length and the operation temperature. The lasing mode varies with the driving current.

At low current, the ground state lasing threshold is reached. By increasing bias currents, the excited state lasing threshold is reached. Then two-state lasing transitions occurred. Finally, lasing transition switches completely changed to the excited state transition. At room temperature, the QD lasers have two lasing transitions at a range of specific cavity lengths. At the critical cavity length (1100 μm), we observed a switching of the lasing wavelength from

the ground state to the excited state transition. The excited state lasing threshold did not occur when the cavity length was longer than 1500 μm. The ground state (excited state) lasing threshold increases (decreases) with increasing operation temperature.

4.6 The Wavelength Switching Transition in Coupled-cavity Lasers 4.6.1 Introduction

Two state lasing of QD lasers had been observed and described in above section. In laser operations, either ground or excited state transition can be the dominant lasing mode depending on the cavity length, the dot density, the driving current, and other structural parameters [50-56]. Once the laser fabrication is completed, the control of the lasing mode becomes difficult. Whether the lasing mode can be controllably switched from the ground state to the excited state or vice versa is an important and an interesting subject. Zhou, et.al.

have previously studied the switching behavior of QD lasers using a coupled-cavity structure, where one laser section was accompanied by a saturable absorber region [57]. By adjusting the voltage of the absorbing region, a 15 nm change in wavelength or ~20 meV change in energy was obtained. This change, however, is unlikely due to the switching between the ground state and the excited state transitions normally observed in PL spectrum because the energy separation was too small compared to the separation of the two PL emission peaks.

More recently, Markus et. al. reported a clear ground/excited state switching, again using a gain section and an absorber section.[58] A 65 meV switching was obtained. However, in these approaches, an absorber section was always used as the controlling device for mode switching while the gain section was used for providing the laser gain.

In this chapter, we describe and demonstrate a clear ground to excited state switching (around 100 nm or 75 meV) using a two-section quantum dot laser. Both sections were forward biased

and were used as either the gain or the absorbing regions. By adjusting the amount of currents going into each section, switching between the ground state emission, ~1.3 μm, and the excited state emission, ~1.2 μm, was obtained. The energy separation between these two states was 75 meV.

4.6.2 Sample Growth and Device Structure

The QD lasers were grown on (100)-n+GaAs substrates using molecular beam epitaxy. The epitaxial layers consist of, starting from the bottom, a 1.2 μm thick n-type Al0.4Ga0.6As cladding layer with 3x10¹⁷ cm⁻³ Si doping, a 0.4 μm thick undoped GaAs waveguide layer, a 1.2 μm thick p-type Al0.4Ga0.6As upper cladding layer with 3 x10¹⁷ cm⁻³ Be doping, and finally a 200 nm thick p+-GaAs contact layer. The active region consists of six InAs QD layers, which reside in the middle of the waveguide layer. Each QD layer is capped by a 6 nm In0.15Ga0.85As layer and a 40 nm GaAs barrier layer is inserted between the QD layers. the in growth rate is 0.056 and 0.15 um/hr for QDs and 6 nm In0.15Ga0.85As layer, respectively.

Conventional ridge waveguide structure was used for laser fabrication. The laser cavity, however, was electrically divided into two sections separated by a 5 μm gap. The top contact layer and the upper p-type cladding layer in the gap were removed by chemical etching to ensure good electrical isolation between the two sections. The structure of the device is schematically shown in Fig. 4.7.

I₁ I₂

I₁ I₂

FIG. 4.7. Schematic of the two-section laser structure.

4.6.3 Result and Discussions

The fabricated laser waveguide had a width of 20 μm. The lengths of the two sections were 300 μm and 850 μm. By injecting different amount of currents in the two sections, we were able to control the lasing mode. Either section can be used as the gain region or the absorbing region. In other words, either section can be used to control the mode switching.

Pure ground state lasing, excited state lasing or both state lasing are achievable by merely adjusting the current ratio of the two sections.

Fig. 4.8 shows the lasing characteristics as functions of the currents injected into the two sections, the horizontal axis being the current injected into the shorter section and the vertical axis being the current injected into the longer section. The emission wavelength of the ground state transition is 1294 nm while that for the excited state transition is 1200 nm. Different lasing modes take place in different regions in this two-dimensional plot. The boundary of the data in each region indicates the threshold condition. Clearly two boundaries can be identified, one for the ground state lasing and the other for the excited state lasing. The allowed lasing region is where the currents are higher than the boundary. The overlap area of the two allowed regions is where dual state lasing takes place. We notice that the threshold boundaries bend inwards at high currents, especially for the ground state threshold curve. In other words, if we increase the current injected into one of the sections, the current going to the other section needs to be increased also in order to reach the threshold condition. This is in contrary to what one would expect because the optical gain in each section should increase with the injection current. However, when we map the threshold condition shown in Fig.4.8, the currents used are relatively high compared to what one would normally use to operate a laser. The effect of heating causes the gain to drop because of the Fermi distribution is a function of temperature.

So higher currents are needed in order to reach the threshold. This phenomenon is particularly obvious for the ground state lasing. This is because the gain at a lower lasing energy is

affected more by the Fermi function, causing the saturated gain to drop.

This current dependent lasing mode distribution can be understood as follows: The gain due to ground state transition is limited (by the dot density) and saturates at high currents. If the cavity loss is low, the ground state will lase first. But if the current injected to one of the section is not enough or even below transparency, the gain required from the other section has to be increased in order for the total loop gain to reach the threshold condition. This required gain may go beyond the saturation value of the ground state. So in this case the ground state can not lase. The only state can lase is the excited state, which, because of a higher density of states, can provide a higher gain as long as one of the section has a high enough current. This is why that the sole excited state lasing occurs when one section has a very high current while the other one has a low current. The dual state lasing happens when currents to both sections are high and it happens in a very wide range. The origin of dual state lasing has been discussed previously and has been attributed to partial clamping of the carriers in the ground state after threshold [59-62].

Based on the result shown in Fig.4.8, one can clearly see that wavelength switching among various QD lasing modes is possible through the variation of currents applied to the

Based on the result shown in Fig.4.8, one can clearly see that wavelength switching among various QD lasing modes is possible through the variation of currents applied to the