Kent Dennis Choquette

GaAs vertical-cavity surface emitting lasers fabricated by reactive ion etching

GaAs quantum well vertical-cavity surface emitting lasers fabricated using low damage reactive ion etching are discussed. Lasers which are partially and completely etched through their structure are compared. The surface recombination velocity of exposed GaAs is not exacerbated in deep etched lasers; other loss mechanisms in shallow etched lasers have comparable impact on laser performance. Etched lasers exhibit low voltage and small differential series resistance at threshold, while devices fabricated by a combination of etching and ion implantation possess lower threshold current. It is found that reactive ion etching has little additional effect on laser operation, whereas the different device structures considered do influence laser performance.<<ETX>>

Vertical-cavity surface-emitting laser diodes fabricated by in situ dry etching and molecular beam epitaxial regrowth

The authors report the first buried active region vertical-cavity surface-emitting laser diodes fabricated using in situ dry etching and molecular beam epitaxial regrowth. The laser emissions of the etched/regrown devices persist over a greater current range and exhibit maximum output powers larger than air-post lasers. The lasers are anisotropically etched into the lower monolithic distributed Bragg reflector using an electron cyclotron resonance SiCl/sub 4/ plasma etch. After transfer in ultra-high vacuum, epitaxial AlGaAs current blocking layers are regrown around the etched mesas. Polycrystalline deposition on the SiO/sub 2/ mask is removed by reactive ion etching to allow electrical contact and top surface emission. The etched/regrown laser characteristics demonstrate efficient current confinement and low thermal impedance. The vacuum integrated processing described offers the prospect of further device performance enhancements and greater functionality.<<ETX>>

GaAs surface reconstruction obtained using a dry process

We report attaining Ga‐terminated (4×2) surface reconstruction on virgin GaAs substrates using a completely dry process at temperatures below the oxide sublimation temperature and without group V overpressure. The native oxides are removed with an electron cyclotron resonance hydrogen plasma treatment, followed by annealing at 500 °C in ultrahigh vacuum, which yields a reconstructed surface suitable for epitaxial overgrowth. Characterization by secondary ion mass spectroscopy and transmission electron microscopy reveals the complete removal of O, reduced C, and high structural order at the epilayer/substrate interface when this preparation method is used before molecular beam epitaxy. Annealing the substrate at a lower temperature yields a nonreconstructed surface possessing significant impurity concentrations, and leads to dislocation defects at the epilayer/substrate interface.

INTERFACIAL CHARACTERISTICS OF ALGAAS AFTER IN SITU ELECTRON CYCLOTRON RESONANCE PLASMA ETCHING AND MOLECULAR BEAM EPITAXIAL REGROWTH

Regrown/processed AlGaAs interfaces using secondary ion mass spectrometry, cross section transmission electron microscopy (TEM), and reflection high energy electron diffraction have been characterized. Two sets of samples, GaAs/Al0.4Ga0.6As (with GaAs on top) and Al0.4Ga0.6As/GaAs (with Al0.4Ga0.6As on top), are used as starting materials. For the GaAs/Al0.4Ga0.6As samples that are first exposed to atmosphere, the experiment is performed in an integrated processing system where etching and regrowth chambers are linked together by ultrahigh vacuum transfer modules. The etching process includes electron cyclotron resonance (ECR) hydrogen plasma cleaning of GaAs native oxides, ECR SiCl4 plasma anisotropic deep etching into Al0.4Ga0.6As, and an optional, brief Cl2 chemical etching. Regrowth is carried out using solid‐source molecular beam epitaxy (MBE). Despite the in situ processing, significant amounts of C, Si, and O impurities at the 10, 5, and 50×1012 cm−2 levels exist at the interfaces. However, the imp...

In-situ process for AlGaAs compound semiconductor: materials science and device fabrication

Processing of III-V compound semiconductor devices in an ultra-high vacuum or a controlled environment has received much attention during the past few years. Major advantages ofn- situ processing include the preservation of pristine material surface, improved device performance, and fabrication of novel devices. This paper reviews anin- situ process compatible with molecular beam epitaxy (MBE) with emphasis on the removal of oxides and surface contaminants from air-exposed GaAs and AIGaAs. We have characterized deep-etched and MBE regrown AIGaAs with the etching achieved using electron cyclotron resonance plasma treatment. A buried heterostructure vertical-cavity surface emitting laser diode fabricated using thisin- situ process is presented.

Vertical-cavity surface-emitting lasers fabricated by vacuum integrated processing

The authors report on the fabrication of vertical-cavity surface-emitting lasers (VCSELs) using vacuum processing techniques. The upper monolithic distributed Bragg reflector around the laser cavity is dry etched down to the top of the active region, followed by in situ contact deposition on the mesa sidewall, providing a short current path through the p-type mirror. These etched VCSELs exhibit lower series resistance, lower threshold voltage, greater thermal dissipation, and higher maximum output power than conventional planar VCSELs made from the same material.<<ETX>>

Molecular beam epitaxy growth of AlGaAs/GaAs vertical cavity surface emitting lasers and the performance of PIN photodetector/vertical cavity surface emitting laser integrated structures

An all-epitaxial planar top emitting AlGaAs/GaAs multi-quantum well laser is fabricated and characterized. The constructed vertical cavity surface emitting laser (VCSEL) consists of GaAs/Al0.2Ga0.8As (100/80 A) quantum wells sandwiched between two doped distributed Bragg reflectors characterized by a two-step composition profile. Two Ga and two Al cells are used to facilitate the growth of mirror profile. The gain-guided VCSEL is found to generate continuous wave at a characteristic temperature of 210°K up to 90°C, and can be amplitude modulated at frequencies above 5 GHz. Thresholds as low as 2 mA, and a CW power more than 1.5 mW, are obtained at room temperature. Monolithic integration of a PIN photodetector on top of the VCSEL is demonstrated and discussed. The integrated photodetector shows an effective linear responsivity to the laser emission of 0.25 A/W.

In situ deposition of Au on plasma-prepared GaAs substrates

In situ deposition of single crystal epitaxial and textured polycrystalline gold films on plasma-cleaned or plasma-etched GaAs substrates is accomplished in an ultrahigh vacuum integrated processing facility. Au/GaAs samples are characterized using reflection high energy electron diffraction, Auger electron spectroscopy, and ion channeling. Au crystallinity in films deposited at 100° C is shown to strongly depend on the GaAs surface cleanliness after plasma processing. Heating the substrate to 250° C after plasma processing subsequently yields epitaxial Au films; omitting the heating procedure results in polycrystalline Au films. The substrate thermal treatment removes residual physisorbed gas molecules and reaction products from the GaAs surface. Epitaxial Au films contain significantly less Ga and As on the free surface of Au than polycrystalline films, and no interaction between epitaxial Au and GaAs is observed.

Monolithic integration of a photodetector and a vertical-cavity surface-emitting laser

Summary form only given. Monolithic integration of a photodetector with a vertical-cavity surface-emitting laser (SEL) is reported. The SELs lase at room temperature and emit a 850-nm highly coherent, low-divergence, circular beam directly from the top surface. The integrated photodetector shows a linear response to the laser emission with an effective responsivity of 0.25 A/W. The SEL consists of a GaAs multiquantum-well active region and doped Al/sub 0.15/Ga/sub 0.85/As/AlAs distributed Bragg reflectors (DBRs). Proton implantation was used to fabricate planar gain-guided lasers. Since the GaAs substrate is opaque to the lasing wavelength, the top mirror is designed to have lower reflectivity and annular contacts are used to permit emission from the top surface. After growing the SEL, additional i-GaAs and n-AlGaAs layers were grown on top of the p-doped DBR to form the p-i-n photodetector (PD). >

Reactive Ion Etching GaAs Vertical-cavity Surface Emitting Lasers

A vertical-cavity surface emitting laser (SEL) consists of a laser diode between two highly reflective mirrors. SELs incorporating distributed Bragg reflectors (DBR) surrounding quantum well active layers and fabricated by ion implantation and dry etching have been demonstrated.1 Monolithic integration of photodiodes using these techniques has also been accomplished.2 A promising device structure is a buried active region with larger bandgapbower index material regrown around the SEL. Fabrication of these and other SEL device configurations requires a simple, low damage, and anisotropic etching technique. We have investigated a low damage reactive ion etching (RIE) process3 and have applied it to GaAs SEL fabrication. The laser material is grown by molecular beam epitaxy and is composed of a lower n-type DBR (AlGaAs/AlAs multilayers), an undoped graded index active region with three GaAs quantum wells, and a top p-type DBR,2 as sketched in Figure 1. Gold circular dots (5, 10, and 20 pm diameter) and rings (inner/outer diameter of 10/20 pm) are defined to serve as etch masks as well as electrical contacts. A Si02 window protects the center of the rings during etching. A PlasmaLab diode reactor is used where the powered lower electrode is covered by a quartz plate and is cooled to -15°C. RIE with 10 sccm of S i c 4 at a pressure of 5x10-3 torr and RF (13.56 MHz) power density of 0.16 W/cm2 yields an etch rate of approximately 0.1 pm/min. A selfbias of 50 volts develops during the etch. We have found these conditions yield ion-stimulated etching exhibiting anisotropic vertical sidewalls with low ion damage.3 As shown in Figure 1, the shallow etches (2.4 pm) are nearly through the bottom of the top DBR, while for the deep etches (7.0 pm) the entire laser structure is etched through. No current confinement (e.g. by ion implantationlY2) is used for the shallow etched lasers. Pulsed and cw emission is measured from the front side of the wafer for the ring-contact lasers, while scattered light from the pillar sides of the metal-topped dot SELs is monitored under pulsed conditions. The threshold current, Ith, under pulsed conditions (200 nsec pulses at 100 kHz) for dot and ring SELs fabricated from the same wafer is plotted in Figure 2. Notice Ith varies approximately linearly with laser diameter indicating that loss mechanisms strongly influence the operation of these lasers. From Figure 2 we observe the loss for the shallow and deep etched SELs are comparable under pulsed excitation. Loss in the shallow lasers arises from diffraction due to the index variation along the laser cavity and from current spreading away from the laser active region. Nonradiative recombination dominates the loss of the deeply etched SELs due to the high surface recombination velocity, s, of GaAs. Assuming