K. N. Kang

Stability of sulfur-treated InP surface studied by photoluminescence and x-ray photoelectron spectroscopy

The degradation behavior of the sulfur-treated InP surface at relatively low temperature has been investigated with x-ray photoelectron and photoluminescence (PL) spectroscopy. The results showed that the treated surfaces were oxidized to In2O3, InPO3, and InPO4 at 250 °C and in a vacuum of 10−3 Torr for 20 min. As the holding time for S-treated InP under a vacuum of 10−3 Torr increased, the PL peak caused by the band edge transition decreased without the formation of oxides. It was therefore suggested that the decrease of the PL intensity for S-treated InP is only related to the generation of phosphorous vacancies at the surface, not to oxide formation. The usefulness of a thin S overlayer on III–V semiconductors was also discussed.

Electrical characteristics of an optically controlled N-channel AlGaAs/GaAs/InGaAs pseudomorphic HEMT

Electrical characteristics of an n-channel Al/sub 0.3/Ga/sub 0.7/As/GaAs/In/sub 0.13/Ga/sub 0.87/As pseudomorphic HEMT (PHEMT) with L/sub g/=1 /spl mu/m on GaAs are characterized under optical input (P/sub opt/). Gate leakage and drain current have been analyzed as a function of V/sub GS/, V/sub DS/, and P/sub opt/. We observed monotonically increasing gate leakage current due to the energy barrier lowering by the optically induced photovoltage, which means that gate input characteristics are significantly limited by the photovoltaic effect. However, we obtained a strong nonlinear photoresponsivity of the drain current, which is limited by the photoconductive effect. We also proposed a device model with an optically induced parasitic Al/sub 0.3/Ga/sub 0.7/As MESFET parallel to the In/sub 0.13/Ga/sub 0.87/As channel PHEMT for the physical mechanism in the drain current saturation under high optical input power.

Carrier lifetimes in dielectric cap disordered GaAs/AlGaAs multiple quantum well with SiN capping layers

Time resolved photoluminescence (PL) characteristics of a SiN cap disordered GaAs/AlGaAs multiple quantum well (MQW) structure exhibit a decrease in carrier lifetime in conjunction with an increase in quantum well disordering (QWD) as the SiN capping layer thickness is increased. The decrease in carrier lifetime is attributed to enhanced carrier trapping due to the defects introduced during dielectric cap quantum well disordering and the relaxation of the momentum conservation during radiative recombination by QWD. Potential applications of these effects on high speed optical devices such as laser diodes (LD’s) and optical modulators are discussed.

High photoresponsivity of a p-channel InGaP/GaAs/InGaAs double heterojunction pseudomorphic modulation-doped field effect transistor

In this letter, we report the electrical and optical characteristics of p-channel In0.13Ga0.87As double heterojunction pseudomorphic modulation-doped field effect transistor (MODFET) structure grown by gas source molecular beam epitaxy. The Hall mobility and the density of 2-DHGs (two-dimensional hole gases) in the pseudomorphic In0.13Ga0.87As channel were measured to be 250 cm2/V s and 1.9×1012 cm−2 at 300 K, and 5800 cm2/V s and 1.5×1012 cm−2 at 23 K, respectively. The fabricated p-channel MODFET shows a good mobility property which is due to high valence band discontinuity of InGaP/GaAs/InGaAs double barriers. The peak energy in the photoluminescence spectrum from the p-channel pseudomorphic MODFET structure was found to be 1.4 eV (λ=881 nm). The photoresponsivity with this modified pseudomorphic MODFET structure shows outstandingly better than that of a pin photodiode, particularly at low incident optical power.

Dielectric cap disordering of GaAs/AIGaAs multiple quantum well by using plasma enhanced chemical vapour deposited SiN capping layer

The multiple quantum well (MQW) structure is a good candidate for the monolithic integration of lasers and waveguides to realize photonic integrated circuits (PIC). For such an application, certain types of technique are needed to define different bandgaps at the active and passive part of the PIC. Quantum well disordering techniques [1-9] have been used to locally disorder the MQW structure without any regrowth and/or selective growth step. Since the disordering of MQW results in a blue shift in bandgap and an increase in refractive index near the bandgap, these techniques can be used to fabricate lasers and waveguides monolithically with only onestep epitaxial growth. There have been several techniques to selectively disorder III-V compound semiconductor QW structures, such as impurity induced disordering [1, 2], ion implantation disordering [3,4], and dielectric cap disordering [5-9]. Among these techniques, since the dielectric cap disordering technique introduces a much lower optical loss compared to the impurity induced disordering and ion implantation techniques which give high optical losses due to high number of defects and high doping concentrations introduced during the disordering process, this technlque is thought to be better suited for fabricating high performance optical waveguide devices. Since a SiO2 Capping layer induces a relatively larger blue shift than a SiN capping layer in a GaAs/A1GaAs QW system, SiO2 is generally used to promote disordering while SiN is used as a mask to prevent bandgap shifts in the capped areas of the devices [5], even though SiN has been used to promote disordering in InGaAs/InGaAsP QW systems [6]. In this study, a dielectric cap disordering of GaAs/ A1GaAs MQW structure using a PECVD SiN capping layer is reported. The blue shift increased together with the thickness of the SiN capping layer as reported for a SiO2 capping layer [7], the amount of blue shift being even larger than a SiO 2 capped sample. We use a GaAs/A1GaAs MQW laser structure which is grown by a metal organic chemical vapour deposition (MOCVD) technique on a Si-doped n ÷ GaAs substrate. The vertical structure of the MQW

Enhanced disordering of GaAs/AIGaAs multiple quantum well by rapid thermal annealing using plasma enhanced chemical vapour deposited SiN capping layer grown at high RF power condition

The multiple quantum well (MQW) structure is a good candidate to monolithically integrate lasers and waveguides for realizing photonic integrated circuits. For such an application, selective disordering techniques are required, because these can make it possible to define a waveguide section laterally by inducing local changes in absorption and refractive indices of a MQW structure, which is needed to fabricate waveguide devices including laser. There are several techniques to selectively disorder the I I I -V MQW structures. These are impurity induced disordering [1,2], ion implantation disordering [3, 4], and impurity-free disordering [5-12] followed by thermal treatment. Impurity induced disordering and ion implantation disordering can disorder MQW structure perfectly, but these techniques introduce many defects and high doping concentrations which may deteriorate the performances of waveguide devices because of losses from scatterings by defects and free carrier absorption in the waveguide. Impurity-free techniques employ dielectrics, such as SiO2 or SiN, a capped annealing technique, which employs rapid thermal annealing (RTA) [5-10] or sealed ampoule annealing with an As overpressure [11, 12]. Impurity-free techniques do not disorder MQW structure completely, but disorder it enough to fabricate waveguides and lasers [9-12]. For impurity-free disordering, there are various film growth techniques, such as chemical vapour deposition (CVD) [11], e-beam evaporation [6, 8, 9], sputtering [5], and plasma enhanced chemical vapour deposition (PECVD) [7, 10, 12], which can deposit SiN and/or Si te films on a MQW structure. The behaviour of disordering is affected by the film quality used in the impurity-free disordering technique. Therefore, if an optimum film growth condition can be found by varying process conditions, then selective disordering can be achieved with the same material without surface degradation which should be accompanied in the disordering process without an encapsulation. The characteristics of film deposited by the PECVD technique can be varied by varying process conditions, such as substrate temperature, ratio of reactant gas, and RF power [13]. As a first step to finding a process-dependent impurity-free disordering, we varied the RF power. We carried out impurity-free disordering using a PECVD SiN cap layer with RTA. We found that this technique could disorder the GaAs/A1GaAs MQW structure and the disordering was enhanced by using SiN film grown at high RF power. We used a GaAs/A1GaAs MQW laser structure which was grown by a metal organic chemical vapour deposition (MOCVD) technique on Si-doped n ÷ GaAs substrate. The structure has the following layers from the top of the substrate: 0.5/xm of n (1018 cm -3) GaAs buffer, 1/zm of n (1017 cm -3) A10.4~Ga0.s3As, 0.1/xm of undoped A10.24Ga0.76As, four 7 nm undoped GaAs quantum wells with 10 nm A10.24Ga0.76As barrier, 0.1/xm of undoped A10.24Ga0.76As, 1/zm of p (1017 cm -3) A10.47Ga0.53As, and 0.2/xm of p+ (10 ~8 cm -3) GaAs. In the substrate design, separate confinement design is chosen to enhance the optical confinement in the quantum wells. SiN films were deposited by a PECVD technique for cap layers. We used dilute silane (5% Sill4 in N:) and high purity NH 3 (99.999%) and N2 (99.9999%) as a reactant gas. During plasma deposition, the ratio of partial pressure (PNH3/Psi~,) was kept at 1, total pressure was kept as 120 Pa by adding N2 gas and the substrate temperature was 300 °C. SiN films were grown with various RF powers (0 W, 60 W and 90 W). The thicknesses and refractive indices, for each RF power condition, were determined by an ellipsometer (Gaertner, Ll17) and a surface profiler (Tencor, alpha step 200). Disordering of MQW samples was accomplished by RTA at 850 °C for 35 s in Ar atmosphere with a heating rate of 65 °C/s. Disordering of MQW

THERMAL STABILITY OF SULFUR-TREATED INP INVESTIGATED BY PHOTOLUMINESCENCE

The effect of sulfur (S) treatments on InP is investigated by low‐temperature photoluminescence (PL) measurements. For both n‐ and p‐InP, the PL intensity is observed to increase about four times in magnitude if the scattering by the S overlayer is relatively small. Some PL bands are observed to disappear after S treatments and then reappear if the S‐treated surface is heat treated at 220 °C in a vacuum of 10−3 Torr. By observing their dependence on the excitation power density, the doping level of the samples, and measurement temperature, these PL bands are ascribed to the optical transitions via surface states. Our results thus indicate that the S‐treated InP surface may not be stable at a subsequent processing temperature of about 250 °C.

Dielectric cap quantum well disordering for band gap tuning of InGaAs/InGaAsP quantum well structure using various combinations of semiconductor-dielectric capping layers

The spatially selective band gap tuning of quantum well structure has been an essential tool to realize the integration of optoelectronic or photonic devices on a single wafer [1, 2]. Quantum well disordering (QWD) techniques are widely employed for the band gap tuning, which induces a change of shape of the quantum well profile by intermixing the well and the barrier materials during annealing. Among the QWD techniques Dielectric cap QWD (DCQWD) method uses the vacancies created both in the dielectric capping layer and at the dielectric-semiconductor interface. There have been many studies of dielectric capping layers, such as SiO2[3], SiNx [4, 5], SrF2[6], and WNx [7], to control the QWD. In this study, DCQWD method was employed for band gap tuning. We were concerned about the effect of the various semiconductor-dielectric capping layer combinations on the band gap tuning as a function of the annealing temperature. The vertical structure of the samples used in this study is schematically shown in Fig. 1. We employed chemical beam epitaxy (CBE) to grow the undoped In0.53Ga0.47As/InGaAsP (Q1.25) single quantum well and the subsequent semiconductor capping layers. The CBE growth temperature was 500 ◦C. The single quantum wells (In0.53Ga0.47As/InGaAsP (Q1.25)) capped with different semiconductor capping layers (InP, In0.53Ga0.47As and InGaAsP (Q1.25)) with 50 nm-thick were prepared, and each was then subjected to topmost capping with dielectric material. The dielectric layers of SiO2 and SiNx (300 nm thick and 100 nm thick, respectively) were deposited by plasma enhanced chemical vapor deposition technique. For SiNx capping layers, dilute Silane (5% SiH4 in N2, flow rate of 40 sccm) and NH3 (99.999%, flow rate of 25 sccm) were used as reactant gases, while Silane (5% SiH4 in N2, flow rate of 40 sccm) and N2O (99.999%, flow rate of 40 sccm) were used for SiO2 layer. The process pressure and temperature were maintained at 0.9 torr and 200 ◦C, respectively. The RF power was 30 W. The growth time was 6 min 40 s for SiNx , and 3 min for SiO2. The refractive index of SiNx film was 1.91. The band gap of virgin quantum well prior to capping was 0.8 eV (λPL = 1550 nm) at room temperature. Thermal treatment was accomplished in N2 atmosphere at various annealing temperatures (600 ◦C– 800 ◦C) for 8 min. PL measurement was carried out at room temperature to identify the degree of band gap tuning. Fig. 2 shows the energy shifts achieved by the various annealing temperatures. The maximum energy shift occurred at 800 ◦C for all samples studied in this work. The energy shift generally increased with the increase of the annealing temperature, which has been reported previously [8]. Fig. 3 shows the blue shift difference between SiNx capped samples and SiO2 capped samples. In cases of InP and InGaAsP semiconductor capping layers, the difference amounted to about 135 meV after annealing at 800 ◦C, while the difference for InGaAs reached about 80 meV. As a result of interdiffusion of atoms between quantum well and barrier, a change in band gap width of the quantum well may happen when the structure is heated above a specific temperature. In our work, the threshold temperature was around 750 ◦C. The overall experimental results of this study, which can be seen in Fig. 2, is that SiNx capping layer caused larger blue shifts than SiO2 capping layer, which means that SiNx layer has many more vacancies than SiO2 layer. However, there was another report for band gap tuning of InGaAsP/InP

Control of the Intermixing of InGaAs/InGaAsP Quantum Well in Impurity Free Vacancy Disordering by Changing NH 3 Flow Rate During the Growth of SiN x Capping Layer

The dependence of impurity free vacancy disordering (IFVD) of InGaAs/InGaAsP QW structure on the characteristics of dielectric capping layer was studied using SiN x film as capping layers. The characteristics of the SiN x capping layer were varied by changing the NH 3 flow rate during SiN x deposition by plasma enhanced chemical vapor deposition (PECVD). The degree of quantum well intermixing (QWI) with SiN X capping layer grown at higher NH 3 flow rate was larger than that with SiN x film grown at lower NH 3 flow rate. This implies that QWI can be easily controlled by simply changing the reactive gas ratio in the growing process of SiN, capping layer. It was also shown that this method to control QWI is better than the method of using two different capping layers such as SiN x film and SiO 2 film in order to get spatially selective QWI on the same substrate.

Thick AlxGa1−xAs: An intrinsically percolating barrier owing to its microscopic structural inhomogeneity

A significant charge transfer, which differs from tunneling, over thick AlxGa1−xAs barrier in GaAs/AlxGa1−xAs asymmetric double quantum wells is studied by cw photoluminescence excitation (PLE) and time‐resolved photoluminescence. It is found that 300‐A‐thick Al0.3Ga0.7As barrier is universally ‘‘leaky’’ with transport time of ∼300 ps, while AlAs and AlAs/GaAs digital alloy barriers with same thickness are not. Aided by a model calculation, we suggest that the intrinsic inhomogeneities in the alloy, which recent x‐ray and scanning tunneling microscope studies revealed, may be responsible.