Mesa Sidewalls Research Articles

Investigation of 3C-SiC Lateral Growth on 4H-SiC Mesas

In this work we report on 3C-SiC heteroepitaxial growth on 4H-SiC(0001) substrates which were patterned to form mesa structures. Two different deposition techniques were used and compared: vapour-liquid-solid (VLS) mechanism and chemical vapour deposition (CVD). The results in terms of surface morphology evolution and the polytype formation using these growth techniques were studied and compared. It was observed both 4H lateral growth from the mesa sidewalls and 3C enlargement on top of the mesas, the former being faster with CVD and VLS. Only VLS technique allowed elimination of twin boundaries for proper orientation of the mesa sidewalls.

Control of Inclined Sidewall Angles of 4H-SiC Mesa and Trench Structures

The sloped sidewall angle in 4H-SiC mesa structure could be controlled by a thermal etching at 900oC in chlorine (Cl2) based ambience. 4H-SiC C face with 8o off substrate was used. The SiO2 layers for the etching mask were formed by a deposited SiO2 layer or a thermally oxidized layer. Thermal etching was carried out at Cl2 ambience at 900oC for 15 minutes. The surface morphologies of the mesa structures were observed with the scanning electron microscope (SEM) and atomic force microscope (AFM). The sloped angles at the mesa sidewalls using deposited SiO2 mask and thermal SiO2 mask were about 23o and 60o, respectively. These results mean that the angle of sloped sidewall can change by mask fabrication method.

AlN/GaN-Based MOS-HEMT Technology: Processing and Device Results

Process development of AlN/GaN MOS-HEMTs is presented, along with issues and problems concerning the fabrication processes. The developed technology uses thermally grown Al2O3as a gate dielectric and surface passivation for devices. Significant improvement in device performance was observed using the following techniques: (1) Ohmic contact optimisation using Al wet etch prior to Ohmic metal deposition and (2) mesa sidewall passivation. DC and RF performance of the fabricated devices will be presented and discussed in this paper.

GaN-Based LEDs with High Emission Directionality Using Photonic Crystal Structures for Sidewall Reflection and Light Extraction

GaN-based light emitting diodes with photonic crystal at the light emitting mesa and nano-hole at the periphery of the mesa using the same photonic crystal pattern are designed and fabricated. Optical properties such as luminescence-current characteristics, radiation profiles and angular spectra of the three kinds of devices (planar, photonic crystal on mesa surface, photonic crystal on mesa surface and sidewall nano-hole arrays) are considered. We demonstrate that the output power enhancement is primarily due to the photonic crystal but the effect on radiation profile is nearly omnidirectional. On the other hand, the sidewall nano-hole array contributes to the optical intensity at the surface normal direction by redirect the lateral emission of the device. By proper design of the photonic crystal pattern, high directionality and high extraction LEDs can be achieved.

Light output enhancement of GaN-based flip-chip light-emitting diodes fabricated with SiO2/TiO2 distributed Bragg reflector coated on mesa sidewall

We report on the enhanced light output of GaN-based flip-chip light-emitting diodes (LEDs) fabricated with SiO2/TiO2 distributed Bragg reflector (DBR) on mesa sidewall. At the wavelength of 400 nm, five pairs of SiO2/TiO2 DBR coats on the GaN layer showed a normal-incidence reflectivity as high as 99.1%, along with an excellent angle-dependent reflectivity. As compared to the reference LED, the LED fabricated with the DBR-coated mesa sidewall showed an increased output power by a factor of 1.32 and 1.12 before and after lamp packaging, respectively. This could be attributed to an efficient reflection of the laterally guided mode at the highly reflective mesa sidewall, enhancing the subsequent extraction of light through the sapphire substrate.

Study of Surface Treatments on InAs/GaSb Superlattice LWIR Detectors

We report on the comparison of mesa sidewall profiles of InAs/GaSb strained-layer superlattice (SLS) detector structures (λ 50% cutoff ≈ 14 μm at V bias = 0 V and T = 30 K) obtained after (a) a conventional BCl3-based inductively coupled plasma etch, (b) a chemical etch (H2O2:HCl:H2O, 1:1:4), and (c) a combination of both etches. We found that the smoothest sidewall profile with reasonable undercut (~5 μm) was obtained after chemical etch only. The chemical etch was optimized primarily using an n-type GaSb substrate. During this process, numerous chemical etchants were examined. GaSb n-type substrates were chosen for this study in preference over InAs substrates due to their high chemical reactivity and the complicated composition of the native oxide. In addition, SLS detectors are usually grown on GaSb substrates and, after hybridization of the focal-plane array to the readout integrated circuit, the GaSb substrate is etched away using a combination of wet and dry etching techniques. We found that H2O2:HCl:H2O (1:1:4) etching solution provided the smoothest etched surface of GaSb, with a root-mean-square roughness of 1.59 nm.

GaSb for passivating type-II InAs/GaSb superlattice mesas

The lack of stable passivation for mesa sidewalls is hampering the development of infrared focal plane arrays (FPAs) based on type-II InAs/GaSb superlattices (SL). We propose the use of GaSb itself to fill the space between FPA pixels, thus eliminating sidewalls and sidewall currents. In such a structure, the conduction band discontinuity between the SL and the GaSb encapsulant constrains electrons to move vertically between the contacts, while the low, vertical hole mobility causes holes to preferentially move transversely. Hence, this scheme (a) eliminates surface leakage currents, which improves detectivity and (b) reduces the density of excess holes in the active region of the device, which improves Shockley–Reed–Hall and direct recombination lifetimes, thereby the detector operating temperature and figures of merit.

Self-Formation of Specific Pyramidal Planes in 4H-SiC Formed by Chlorine Based Ambience

. Crystallographically specific pyramidal planes in 4H-SiC could be naturally formed at mesa sidewalls by a thermal etching at 900oC in a chlorine (Cl2) based ambience. The mesa structures have been fabricated on the 4H-SiC C-face with 0-45o off-angle toward [11-20]. The etched surface was rather smooth, and bunched step structures on the specific pyramidal planes were not observed with atomic force and scanning electron microscopes. The formation mechanisms of the specific pyramidal planes at mesa sidewalls are discussed on the basis of these experimental results.

Enhanced light extraction of nonpolar a-plane (11-20) GaN light emitting diodes on sapphire substrates by photo-enhanced chemical wet etching

The extraction efficiency of nonpolar a-plane (11-20) GaN LEDs on sapphire substrates has been enhanced by selectively etching the mesa sidewall faces and the n-type GaN surfaces with photoenhanced chemical wet etching. Submicron-sized trigonal prisms having prismatic planes of {1-100} were clearly displayed on the n-type GaN surfaces as well as the sidewall face after 5 min etching at 60 degrees C. The radiation patterns have shown that more light is extracted in all directions and the output powers of surface textured a-plane GaN LEDs have increased by 25% compared with control samples. PEC wet etching produced unique feature of etching morphology on the mesa sidewall faces and the n-type GaN surface.

Degradation Analysis of InP Buried Heterostructure Layers in Lasers Using Optical-Beam-Induced-Current Technique

The degradation of InP buried heterostructure layers in lasers during constant-power aging is investigated by using the optical-beam-induced-current (OBIC) technique. An increase in OBIC intensity after aging is detected in the InP layer, and it is shown that the carrier concentration around the p-type InP buried layer of the mesa sidewall is decreased by aging. This technique is useful for detecting changes in the carrier concentration in InP areas of several micrometers in lasers by using an incident light absorbed in an InP layer.

Effects of Zn Doped Mesa Sidewall on Gain Enhanced InGaAs/InP Heterobipolar Phototransistor

The excellent detectability of the gain enhanced InGaAs/InP heterobipolar phototransistor (GE-HPT) is demonstrated and attributed to a reduction in the reverse leakage current at the base-collector junction and the enhancement of current gain at the emitter-base junction achieved by using a current blocking structure with a Zn doped mesa sidewall. The common emitter grounded current gain agrees well with the photo-conversion efficiency of several tens of thousands of A/W at incident optical powers in the hundred nanowatt to sub-picowatt range over several orders of magnitude. The deep mesa structure in the GE-HPT is also effective in ensuring superior isolation of better than 25 dB between adjacent arrays.

Enhanced Light Extraction in GaN-Based Light Emitting Diodes Fabricated with Increased Mesa Sidewalls

We report on the enhanced light extraction of GaN-based light emitting diodes (LEDs) fabricated with increased mesa sidewalls. It is shown that light extraction and operating voltages in LEDs typically have an inverse relationship that depends on the perimeter of the mesa sidewalls and the mesa area. Based on these observations, we designed an advanced LED incorporating regularly arrayed circular mesa holes to simultaneously maximize the mesa sidewall perimeter and area. Compared to a reference LED, the output power of our mesa-hole LED was enhanced by 12%, and its operating voltage was decreased by 0.1 V, resulting in a power efficiency improvement of 8.8%.

Mid-Infrared GaAs/AlGaAs Quantum Cascade Lasers Technology

The quantum cascade lasers (QCLs) are unipolar devices based on intersubband transitions and tunnelling transport [1, 2]. The GaAs-based QCLs have proved to be an effective source of laser radiation in mid-infrared (MIR) as well as far-infrared (FIR) regions. They find application in the gas sensing systems (e.g., for detecting CO2, NO, CH4) [3], medical diagnostics [4] and environment monitoring [5]. The complex design and operating principle require the technology of quantum cascade lasers to ensure the highest accuracy, uniformity and repeatability of both epitaxy and device processing [6]. High requirements for epitaxy are connected with ultrathin layers of periodic active region, where the wave functions are to be properly engineered [7–9]. The thickness accuracy of the structure should in general be better than 3%. Another crucial issue is the optimisation of injector doping, which is necessary to assure a large enough dynamic range of laser operation [10, 11]. In order to restrict the lateral current spreading in the device it is necessary to form a good quality deep mesa structures. Since the available gain is usually limited, the device design should minimize the radiation losses. The mesa is etched through active region and reduction of the optical losses requires a smooth waveguide sidewalls [12]. There is a number of QCLs designs reported in the literature [6, 13]. Sirtori et al. [6]. have presented the ridge structure and double trench structure, in which ion implantation was used for electrical isolation instead of dielectric layers. The optimum one seems to be the double trench construction which insures a good overlapping of the lateral mode with the gain region and enables stable epi-down mounting necessary for efficient cooling of the device [14]. The construction in which the extended proton-implanted regions were applied in the area defined by the two trenches, was the another presented solution. In this case the channel for current injection was narrower than the ridge width, which minimizes absorption of generated radiation in dielectric layers insulating mesa sidewalls [6, 15]. As a rule the relatively high voltages as well as current densities are necessary to polarize QCLs. That indicates demand for low resistance ohmic contacts in order to reduce the device serial resistance. These contacts should be characterized by thermal stability, low depth of metal diffusion into semiconductor layers and lateral uniformity of metal–semiconductor interfaces. In this paper we discuss some issues fabrication technology of Al0.45Ga0.55As/GaAs QCL operating at λ ≈ 9.4 μm. The devices lased up to 262 K (−11 ◦C) with optical powers over 1 W at 77 K, threshold current density values of 7 kA cm−2 and differential efficiencies above 0.6 W/A. Since lasers were uncoated, the quoted values should be understood.

Gain-Enhanced InGaAs–InP Heterojunction Phototransistor With Zn-Doped Mesa Sidewall

A gain-enhanced InGaAs-InP heterojunction phototransistor (GE-HPT) with a Zn-doped mesa sidewall designed to reduce both base-emitter recombination currents and base-collector dark currents is realized. The developed GE-HPT has an optical conversion efficiency as large as 24 kA/W at an incident optical power of 29 nW and a wavelength of 1.55 mum, which is the highest gain yet reported in HPTs. The measured dark current in the base-collector junction is comparable to that of planar-type PIN photodiodes. The high gain and low dark current characteristics of the phototransistors make them promising for use in a weak light detection system.

Characterization of LWIR diodes on InAs/GaSb Type-II superlattice material

Long wavelength infrared (LWIR) focal plane arrays (FPAs) built on Type-II strained layer InAs/GaSb superlattice materials are emerging as an alternative to LWIR HgCdTe. We have made progress in the development of this technology in a collaborative effort between Raytheon Vision Systems and Jet Propulsion Laboratory, resulting in successful devices with LWIR cutoff wavelengths. We report here two investigations related to wafer processing and superlattice material characteristics. The critical interface between the superlattice and the silicon dioxide passivation was examined at the atomic scale by high resolution transmission electron microscopy (HRTEM), showing a conformal coating on an InAs/GaSb mesa sidewall, which undulates with the superlattice periodicity due to differential etching. Electron energy loss spectroscopy (EELS) showed that oxides of the superlattice elements were present but minimal, and some occasional arsenic precipitates were observed at the passivation interface. Our previous analysis of the current–voltage curves was extended further to reveal the minority carrier lifetimes responsible for producing the generation–recombination (GR) and the diffusion dark currents. Lifetimes at 78 K were found to be 6 and 20 ns in the GR and diffusion processes, respectively. Lifetimes from both mechanisms track together with temperature. A HgCdTe diode was analyzed in the same manner for comparison.

Low-noise GaN ultraviolet p-i-n photodiodes on GaN substrates

We report low-noise GaN visible-blind homojunction p-i-n photodiodes. The devices are grown on a freestanding bulk GaN substrate and are fabricated using a “ledged” surface depletion technique to suppress the mesa sidewall leakage. For an 80-μm-diameter photodetector, the dark current density is lower than 40 pA/cm2. A room-temperature noise equivalent power of 4.27×10−17 W Hz−0.5 and a detectivity of 1.66×1014 cm Hz0.5 W−1 are achieved at a reverse bias of 20 V. The noise performance of the reverse-biased GaN p-i-n photodiodes are among the best values reported to date and demonstrate the potential of GaN photodiodes for low-noise high-speed UV detection.

The growth of GaAs and InAs dots on etched mesas: The effect of substrate temperature on mesa profile and surface morphology on dot distribution

The molecular beam epitaxy (MBE) growth of GaAs and InAs quantum dots on etched mesas has been studied using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The [0 1 1]-oriented mesas are etched into (1 0 0) GaAs substrates, exposing (5 3 3)B sidewall facets. At a substrate temperature of 610 °C a top (1 0 0) plane is seen to evolve on a ridge mesa structure. Alternatively, if the overgrowth is carried out at 630 °C no such facet is seen, and the top ridge remains unchanged during GaAs growth. By controlling the mesa shape, either ordered lines of dots can be grown or the dot density can be varied from <5×10 8 cm −2 to >1×10 11 cm −2 on the same substrate in pre-defined regions. The dot distribution observed on the mesa sidewalls and top is discussed in terms of net migration of adatoms from different facets, underlying step density, step height and surface curvature of the mesa top.

Crystallographic plane dependent Fe and Si dopant incorporation and activation in InP

AbstractVarious crystallographic planes are exposed when growing along mesa sidewalls during metalorganic vapor phase epitaxy. Dopant incorporation and activation on these higher index surfaces can be quite different from that observed on (001) surfaces. Incorporation and activation of Si and Fe in MOVPE grown InP on (001), (110), and (311)A and B surfaces are reported. The Si donor concentration is higher on the P‐terminated B surface than the A surface while the opposite behavior is observed for Fe incorporation. In the fabrication of optoelectronic devices, it must be remembered that it is the carrier concentration, that is, activated atomic species, that is relevant for device performance and not the incorporation of atomic species alone. Activation of Si is &gt;1 on all surfaces; however, Fe is shown to be an effective compensating species only on the (001) and (311)A crystallographic surfaces. Data for the presence of oscillations in the iron concentration on (001) surfaces are also reported. The active Fe concentration is shown to be related to the Fe atomic concentration relative to the donor density on the different crystallographic surfaces, suggesting that Fermi level position sets a limit to the fraction of Fe centers which are electrically active as a deep trap. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

GaN light-emitting diode with monolithically integrated photonic crystals and angled sidewall deflectors for efficient surface emission

In order to obtain efficient surface emission, we propose and demonstrate a GaN light-emitting diode (LED) structure. A two-dimensional photonic crystal (PC) pattern is integrated to the sapphire substrate before the epigrowth by employing laser holography. In addition, angled sidewall deflectors (ASDs) are formed on the mesa sidewalls. Both the PC and ASDs redirect guided photons into the surface-normal direction. When compared to a conventional LED structure, we could obtain a twofold increase in the total surface emission and the surface-normal emission intensity enhanced by a factor of 2.5.

Dry etching characteristics of GaN for blue/green light-emitting diode fabrication

The etch rates, surface morphology and sidewall profiles of features formed in GaN/InGaN/AlGaN multiple quantum well light-emitting diodes by Cl 2-based dry etching are reported. The chlorine provides an enhancement in etch rate of over a factor of 40 relative to the physical etching provided by Ar and the etching is reactant-limited until chlorine gas flow rates of at least 50 standard cubic centimeters per minute. Mesa sidewall profile angle control is possible using a combination of Cl 2/Ar plasma chemistry and SiO 2 mask. N-face GaN is found to etch faster than Ga-face surfaces under the same conditions. Patterning of the sapphire substrate for improved light extraction is also possible using the same plasma chemistry.