In this paper we will study the influence of InGaN underlayer on efficiency of InGaN-based LEDs grown by plasma-assisted molecular beam epitaxy (PAMBE). We observed that LEDs with the thinnest underlayer have the highest efficiency. This finding agrees with the theory that the defects, which are buried in standard LEDs are in fact generated during the growth of GaN in MOVPE at high temperature. In case of PAMBE, the growth temperature of GaN is 300°C lower, and these defects are not generated in the first place and there is no need for an InGaN underlayer.
We present a method of in-plane modification of the refractive index using ion implantation and electrochemical etching of GaN layers. Proposed method allows for the fabrication of embedded air-GaN channels that can be periodically arranged inside III-nitride heterostructures. Importantly, a flat top surface is preserved for further regrowth. High refractive index contrast between air and GaN makes the proposed technology attractive for the fabrication of embedded photonic structures such as diffraction gratings for distributed feedback laser diodes (DFB LDs). We discuss the impact of the different design of air-GaN channels on the properties of DFB LDs.
III-nitride semiconductor system is widely used in many electronic and optoelectronic applications. The presence of extremely high piezoelectric field in quantum wells (QWs) is known to cause severe separation of electron and hole wavefunctions and limits the thickness of QWs used in devices. We have recently shown that wide QWs are also a viable solution in optoelectronic devices. In this paper we will discuss the physics of recombination in wide InGaN QWs. We will show that the piezoelectric field is fully screened in case of wide InGaN QW and light emission occurs through excited states with high wavefunction overlap.
The innovative method of μLEDs fabrication is presented. The light emission area was defined by a size of the tunnel junction (TJ) embedded inside diode. The epitaxial structures were grown entirely by plasma assisted molecular beam epitaxy (PAMBE) on (0001) bulk GaN crystals. The PAMBE grown LED structure emitting light at 450 nm was capped with TJ region and 100 nm n-type GaN. The emission size of μLEDs was defined by ion implantation of n-type GaN and TJ region. The entire surface of the wafer is atomically flat, ready for the next epitaxial process, which is important e.g. for TJ µLEDs red-green-blue displays with a stack of 3 µLEDs.
Incorporation of tunnel junctions (TJs) to device structure enabled vertical integration of multicolor light emitting diodes (LEDs) and laser diodes (LDs). The TJs allows to control the current path in distributed-feedback LDs and micro-LEDs. It opens possibility to design new architecture devices like “inverted” LEDs or LDs with TJs located below active region. These devices have the sequence of p and n type layers similar to structures grown on hypothetical p-type (0001) GaN substrate, which is beneficial for high carrier injection efficiency, and enables operation at cryogenic temperatures. Finally, we also discuss the properties of bi-directional LEDs and wavelength-tunable LEDs.
We present LED profiting from the bottom-tunnel junction (BTJ) construction. The BTJ design aligns the polarization fields in a desired direction in the vicinity of active region and inverts the ordering of the layer stack in the structure. This leads the situation were conductive, n-type layer is on the very top of the structure. Since current spreading in n-type material is much better than in p-type, BTJ-based light emitters open new possibilities in heterostructure design. In this talk we present new light emitting structures grown by plasma-assisted MBE based on BTJ platform and compare prospects for bottom and top tunnel junction devices.
In this paper, we try to resolve problems related to decreasing the size of an LED, and find a solution that would let us preserve optoelectronics parameters. The main idea is to use tunnel junctions to define the current path and, therefore, define the size of µLED. This way, during fabrication, there is no need to etch the active region. That way, it does not introduce any degradation nor problems related to surface states or differences in electrical fields inside the device.
We have fabricated such devices with sizes ranging from 100 µm-5 µm. In the characterization of these devices, it became apparent that, both electrical and optical parameters, are fully scalable with size. Most importantly, we do not observe an increase in the non-radiative recombination coefficient even for the smallest device. In addition, we observe excellent thermal stability of their light emission characteristics.
New approach towards efficient light emission with bottom-tunnel junctions is developed. The bottom-tunnel junction design aligns the polarization fields in a desired direction in the vicinity of quantum well, while simultaneously eliminating the need for p-type contacts, and allowing efficient current spreading. By preventing electron overshoot past quantum wells, it disables carrier recombination in undesired regions of the heterostructures, increasing injection efficiency and opening new possibilities in heterostructure design. InGaN-based buried-tunnel junction is used to construct first monolithically grown p-type-down laser diode on n-type, Ga-polar bulk GaN substrate. Unique advantages of such construction that enables to separate design of carrier injection and optical mode confinement for such laser diode structures is discussed.
In this work we study the growth mechanisms of InGaN in plasma-assisted molecular beam epitaxy (PAMBE). We investigate growth of InGaN layers on vicinal GaN (0001) substrates. Indium incorporation as a function of gallium and nitrogen fluxes was examined. We propose microscopic model of InGaN growth by PAMBE postulating different indium adatom incorporation mechanisms on two nonequivalent atomic step edges of wurtzite crystal. The different roles of gallium and nitrogen fluxes during the growth of InGaN layers is discussed.
We present recent progress in the growth of nitride based laser diodes (LDs) made by Plasma Assisted Molecular Beam Epitaxy (PAMBE). In this work we demonstrate LDs grown by PAMBE operating in the range 450 – 460 nm. The LDs were grown on c-plane bulk GaN substrates with threading dislocation density (TDD) ranging from 103 cm-2 to 104 cm-2. The low TDD allowed us to fabricate cw LDs with the lifetime exceeding 2000 h at 10 mW of optical output power. The maximum output power for 3 LDs array in cw mode was 280 mW and 1W in pulse mode. The low temperature growth mode in PAMBE allow for growth of AlGaN-free LDs with high In content InGaN waveguides. The key element to achieve lasing wavelengths above 450 nm was the substantial increase of the nitrogen flux available during the growth by PAMBE.
In the present work we demonstrate a concept of a "weak plasmonic cladding" for the improved transversal optical confinement in the
structures of nitride lasers diodes emitting in the violet and blue spectral region. We show that by using highly doped GaN:O or
GaN:Si layers we can induced the reduction of the refractive index by around 1-2% comparing to a lightly doped material. Such a
material can be effectively used as optical cladding replacing thick, highly strained AlGaN layers. Plasmonic claddings can be grown
by two methods: first of them is High Nitrogen Pressure Solution growth (an introduced donor is an oxygen) and Molecular Beam
Epitaxy with silicon as a donor. In the both cases we can reach a free carrier concentration of up to around 1020cm-3. MOVPE method
so far did not show capabilities for achieving so high doping level. We demonstrate the use of such layers for the construction of the
violet and blue laser diodes and laser diodes mini-arrays showing a total suppression of the substrate mode leakage.
Metalorganic vapor phase epitaxy (MOVPE) and plasma assisted molecular beam epitaxy (MBE) were used as alternative techniques to fabricate similar group-III-nitride laser structures. Utilization of high-pressure-grown GaN substrates resulted in reduction of threading dislocation density down to 105 cm-2. Light amplification features of the measured structures were evaluated by means of the variable stripe length method. Maximum peak modal gain values of 180 cm-1 for the MOVPE-grown sample and 315 cm-1 for the MBE-grown one were reached at corresponding pump power of 464 kWcm-2. Temperature-dependent photoluminescence measurements yielded activation energies of 41 meV nad 22 meV for MOVPE- and MBE-grown samples, respectively. Saturation lengths of 350 &mgr;m and 250 &mgr;m determined for MOVPE and MBE structures indicate reduced rate of nonradiative recombination compared to heteroepitaxy on foreign substrates. Differences in nonradiative recombination processes between the investigated structures lead to deviations in threshold for stimulated emission in favor of the MBE-grown sample.
We report on the 1.5 μm intersubband absorption measured on GaInN multi-quantum wells with AlInN barriers grown by RF plasma assisted molecular beam epitaxy (PAMBE). The intersubband light absorption was
demonstrated as a function of the well width (1.3 nm - 3 nm) at the wavelength 1.4μm - 2.5 μm. The use AlInN barriers allowed to achieve strain compensated and crack free structures on GaN substrates. The preformed XRD mapping of a and c lattice constants show that AlInN/GaInN MQWs are fully strained and have up to 7% of indium in the barriers. The replacement of AlGaN by AlInN barriers opens new possibility to grow strain compensated crack free intersubband based devices like electooptical modulators and switches operating at telecommunication wavelengths.
Up until recently the successful growth of nitride optoelectronic devices had been demonstrated only with Metal Organic Vapor Phase Epitaxy (MOVPE) due to the possibility of growth of GaN at temperatures which exceed 1000C. Molecular Beam Epitaxy (MBE) seemed to be less adapted for this purpose. It was believed that MBE can give good results only if the growth conditions are close to these used in MOVPE reacotors. Indeed, the first successful growth of
laser diode (LDs) was obtained using ammonia MBE at temperatures as high as 950oC. The new perspective has been opened recently by successful growth of LD structures on nearly dislocation free GaN bulk substrates using Plasma Assisted MBE at much lower temperatures of 590-710oC. The laser structures are deposited on the high-pressure-grown low dislocation bulk GaN substrates taking full advantage of the so called adlayer enhanced lateral diffusion channel for adatoms below the dynamic metallic cover. Devices grown by PAMBE on bulk substrates compare very favorably to the early laser diodes fabricated using the MOVPE technique, providing evidence that the relatively low growth temperatures used in this process pose no intrinsic limitations on the quality of the optoelectronic components. This opened up a number of new possibilities to the low temperature PAMBE including high power blue LDs, green and UV LDs, and quantum cascade lasers.
High-power laser diodes emitting in the violet - UV region are needed for many applications related to data storage, full color laser projectors, pollution screening etc. This type of device is difficult to fabricate by using the presently available technology of epitaxial growth which employs the lateral overgrowth scheme to reduce the dislocation density in the active layer of the device. This paper presents a new generation of wide stripe laser diodes, which structures were coherently grown on bulk, nearly defect free GaN substrates. Thanks to a low and homogeneously distributed dislocation density (3×105cm-3), these devices are able to emit a very large optical power in excess of 2.5 W with a slope efficiency per facet of around 0.3 W/A and threshold current densities of 5-10 kA/cm2. The use of wide 15 μm stripe lowers the optical power density on the mirrors, and helps avoiding their optical damage. We believe that these devices clearly show the potential of homoepitaxy for high-power lasers applications.
The article shows the most important experimental results describing the properties of nitride layers on GaN single crystals. The layers were grown using metal-organic chemical vapor deposition (MOCVD). The growth was monitored by in-situ laser reflectometry. The layers contain very small dislocation density of about 10 to 103 cm$min2 (the same as in GaN substrates). Morphology and crystallographic quality was examined using atomic force microscopy and X-ray diffraction. The layers have excellent photo luminescent properties which have a direct impact on the optoelectronic device properties.
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