Hirotaka Kizuki

Selective metalorganic chemical vapor deposition growth of GaAs on AlGaAs combined with in situ HCl gas etching

Abstract Selective metalorganic chemical vapor deposition (MOCVD) growth of GaAs on Al 0.48 Ga 0.52 As combined with in situ HCl gas etching was investigated. In the case that AlGaAs surface was oxidized prior to the in situ HCl gas etching, accumulation of both oxygen and chlorine were found at the GaAs/AlGaAs regrowth interface. The dislocation density in the regrown GaAs layer was also increased to over 1×10 8 cm -2 by the existence of the accumulated oxygen and chlorine. The high quality GaAs regrown layer on AlGaAs with low dislocation density of 4.2×10 4 cm -2 was obtained by using the GaAs cap layer to prevent the oxidation of AlGaAs surface, and by the adequate AsH 3 flow rate during the HCl gas etching. It was also found that the complete removal of surface oxide on the GaAs cap layer just prior to the HCl gas etching makes perfect reduction of the accumulation of oxygen and chlorine at the GaAs/AlGaAs regrowth interface. The buried ridge waveguide laser fabrication was successfully demonstrated by the selective MOCVD growth combined with the in situ HCl gas etching.

Reduction of interface contamination in regrown GaAs on AlGaAs using a novel two-step HCl gas etching process

A two-step HCl gas etching process followed by regrowth using metalorganic chemical vapor deposition (MOCVD) has been developed. Two-step HCl gas etching involves a low-temperature HCl gas treatment (LTT) followed by high-temperature HCl gas etching. Upon using this process, no carbon accumulation was detected at the interface between the etched Al 0.48 Ga 0.52 As layer and the regrown GaAs layer. Oxygen accumulation at the regrowth interface was reduced to less than one fifth by applying LTT. The dislocation density in the regrown GaAs layer was shown to be comparable to that found in the usual GaAs homoepitaxial layer. The results of carrier lifetime measurements in GaAs/AlGaAs regrown double-hetero (DH) structures indicated the excellent quality of both the regrown GaAs layer and the regrowth interface. It was thus demonstrated that by using this newly developed two-step HCl gas etching process, the crystalline quality of a regrown GaAs layer on AlGaAs can be sufficiently improved to be used as an active layer of optical devices

Observation of quasiperiodic faceting both on MOCVD-grown and on gas-etched surfaces of vicinal (1 1 0)GaAs substrates

Abstract Surface morphology has been studied by means of Nomarski and atomic-force microscopy regarding grown or etched surfaces of vicinal (1xa01xa00)GaAs substrates. The substrates used were misoriented by 2° or 6° towards [ 0 0 1 ], [ 0 0 1 ], [ 1 1 1 ], or [ 1 1 1 ]. The same reactor was used both for metal-organic chemical vapor deposition (MOCVD) growth and for HCl gas etching. The surface showed a variety of morphology, depending on the misorientation and on whether they are grown or etched. Among them, quasiperiodic macrosteps of 15–70xa0nm in height were observed on the grown surfaces as well as on the etched surfaces when the misorientation was towards [ 0 0 1 ]. The edge lines of the macrosteps showed excellent straightness on the grown surface when the substrate was misoriented towards [ 0 0 1 ] by 6°.

Study of initial buffer layer in GaAs-on-Si growth

Abstract An AlAs/GaAs low temperature buffer-layer (LTB) has been found to be effective for reducing the density of pit-defects in the GaAs layer grown on Si, which are the origin of micro-cracks. SEM observation has indicated that the Si surface is covered more smoothly and more uniformly by the AlAs/GaAs LTB rather than by conventional GaAs LTB. Consequently, the pit-defect density was remarkably reduced by employing the AlAs/GaAs LTB and the micro-crack free GaAs-on-Si with a GaAs thickness of 5 μm has been obtained.

Metalorganic chemical vapor deposition growth of high-quality AlGaAs using dimethylethylamine alane and triethylgallium–dimethylethylamine adduct

Abstract High-purity Al 0.25 Ga 0.75 As layer has been successfully obtained by metalorganic chemical vapor deposition (MOCVD) using new precursors of dimethylethylamine alane (DMEAA) and triethylgallium–dimethylethylamine adduct (TEG–DMEA). It is shown that the concentrations of residual carbon and oxygen in the AlGaAs layer grown at a relatively low V/III ratio were under detection limits of secondary-ion mass spectrometry (3.0×10 15 and 5.0×10 15 xa0cm −3 , respectively). In the photoluminescence spectra at 4.2xa0K, the peak intensity of carbon-related emission was much weaker than that of bound-exciton (BE) emission. The full-width at half-maximum for the BE peak was as narrow as 5.6xa0meV. Any predeposition between these precursors due to a gas-phase prereaction was not observed. These results indicate that the combination of DMEAA and TEG–DMEA are promising precursors for MOCVD growth of AlGaAs with low residual impurity concentration.

Large-scale metalorganic chemical vapor deposition growth of highly reliable 780 nm AlGaAs multiple quantum well high-power lasers

Abstract Successful large-scale (twelve 2 inch diamter wafers/run) metalorganic chemical vapor deposition (MOCVD) growth of AlGaAs multiple quantum well (MQW) structure for 780 nm high-power lasers using a barrel-shaped reactor is demonstrated. Excellent uniformity of growth rate (∓3.5%) and Al content (∓0.3%) for an Al 0.48 Ga 0.52 As layer within a 2 inch diameter wafer has been obtained. Very accurate layer thickness control down to 1 nm has also been realized for AlGaAs well layer. Application of this technique to 780 nm AlGaAs MQW high power laser diodes realized excellent uniformity of the laser characteristics. The uniformity of the beam divergency to the junction plane (θ ⊥), lasing wavelength and threshold current were 27∓1°, 785∓3 nm and 48∓7 mA, respectively. In addition, it is shown that the reduction of the oxygen concentration in the laser crystal below 1x10 17 cm -3 is essential to obtain low threshold current with good reproducibility. Stable operation of the laser diodes for more than 1000 h has been also confirmed even at accelarating condition of 60°C, 50 mW.

Crack propagation and mechanical fracture in GaAs-on-Si

This paper describes the study of crack propagation and mechanical fracture in GaAs-on-Si, which are closely related with the residual stress. The crack propagation is often observed as the GaAs thickness exceeds about 3 µm, and the upper limit of the number of cracks increases linearly as the GaAs thickness increases. The cracks propagate from the surface defects, where stress ten times larger than the original residual thermal stress in GaAs-on-Si exists. The mechanical fracture strength (ζ) of the GaAs-on-Si wafer decreases as the GaAs thickness increases, and becomes equal to that of the bulk GaAs at the thickness of about 3 µm due to the concentrated stress near the cracks. The back coating of SiO2 is effective for stress relaxation, and the preliminary result of about 3×108 dyn/cm2 of stress relaxation is obtained.

Time-Resolved Photoluminescence Study on a Hetero Interface Formed by Direct Regrowth of GaAs on an Al0.3Ga0.7As Surface Prepared by an In Situ HCl Gas Etching Process.

We have studied the quality of Al0.3Ga0.7As/GaAs/Al0.3Ga0.7As double hetero (DH) structures, whose lower hetero interface is formed by regrowing GaAs directly on an etched Al0.3Ga0.7As surface. The Al0.3Ga0.7As surface is prepared by two-step in situ etching, which is comprised of a low temperature treatment and high temperature etching with HCl gas, just prior to the regrowth by metalorganic chemical vapor deposition. Time resolved photoluminescence measurements have revealed that the optical quality of the regrown DH structure is significantly improved by applying the two-step HCl gas etching process compared to the wet processed one. The PL decay curve of the in situ processed sample, however, shows a relatively shorter carrier lifetime than that of the continuously grown DH sample. An interface recombination velocity of 6.8×103 cm/s is obtained for the in situ processed AlGaAs interface. This value for the AlGaAs with a high Al content is comparable to the best results for the in situ processed GaAs and InGaAs surfaces reported so far. The relatively high recombination velocity compared to continuously grown interfaces is mainly due to residual oxygen at the in situ processed interface of concentrations as low as 2.2×1011 cm-2.

Quasi-persistent photoconductivity of double-heterojunction pseudomorphic high-electron-mobility transistor epitaxial wafers

Unusual electrical instability caused by light illumination in double heterojunction pseudomorphic high-electron mobility transistor (DH-pHEMT) epitaxial wafers is reported for the first time. The principal parameters of DH-pHEMT, namely, sheet carrier concentration, sheet resistance, and electron mobility, change slowly with time after illumination is shut off at 300 K. The relaxation time of this transit is estimated to be longer than 10 min for some samples, grown by solid source molecular beam epitaxy. This phenomenon is called quasi-persistent photoconductivity (QPPC). The relationship between QPPC and interface roughness is investigated. Through atomic force microscopy measurements on both upper AlGaAs/InGaAs interface and lower InGaAs/AlGaAs interface of the DH-pHEMT, the lower interface of InGaAs channel is found to be much rougher than the upper one. It is concluded that control of interface smoothness of the lower heterointerface is essential in order to obtain high-performance DH-pHEMT epitaxial wafers with high reproducibility.

In situ processing of III–V semiconductors: Mile stones and future prospects

Abstract In situ processing combined with metalorganic vapor phase epitaxy (MOVPE), molecular beam epitaxy, or chemical beam epitaxy appears to be an attractive method for fabricating sophisticated optoelectronic devices such as buried heterostructure lasers, vertical cavity surface emitting lasers, and photonic integrated circuits. Successful reduction of residual contaminants at the regrowth interface and improvement in the optical and electrical quality of the regrown layer has been achieved by using in situ processing techniques. Device fabrication is alrady taking advantage of this kind of technology. Nevertheless, interface quality between an in situ etched layer and a regrown layer has not yet reached the status of continuously grown interfaces. In this paper, progress of in situ processing is reviewed mainly focusing on our recent studies on in situ HCl gas etching in MOVPE. The approach of two-step HCI gas etching has proven superior to obtain clean regrowth interfaces, leading to the conclusion that the in situ processing can be widely used for advanced optoelectronic device fabrication.