Defect-Related Luminescence in Undoped GaN Grown by HVPE

Hydride vapor phase epitaxy (HVPE) is used for the growth of low-defect GaN. We have grown undoped films on sapphire and investigated them using steady-state and time-resolved photoluminescence (PL). One of the dominant PL bands in high-quality GaN grown by HVPE is the green luminescence (GL) band with a maximum at 2.4 eV. This PL band can be easily recognized in time-resolved PL measurements due to its exponential decay even at low temperatures (\u3c50 K), with a characteristic lifetime of 1–2 μs. As the temperature increases from 70 K to 280 K, the PL lifetime for the GL band increases by an order of magnitude. This unusual phenomenon can be explained on the assumption that the electron-capture coefficient for the GL-related defect decreases with temperature as T −2.6

Identification of point defects in HVPE-grown GaN by steady-state and time-resolved photoluminescence

We have investigated point defects in GaN grown by HVPE by using steady-state and time-resolved photoluminescence (PL). Among the most common PL bands in this material are the red luminescence band with a maximum at 1.8 eV and a zero-phonon line (ZPL) at 2.36 eV (attributed to an unknown acceptor having an energy level 1.130 eV above the valence band), the blue luminescence band with a maximum at 2.9 eV (attributed to ZnGa), and the ultraviolet luminescence band with the main peak at 3.27 eV (related to an unknown shallow acceptor). In GaN with the highest quality, the dominant defect-related PL band at high excitation intensity is the green luminescence band with a maximum at about 2.4 eV. We attribute this band to transitions of electrons from the conduction band to the 0/+ level of the isolated CN defect. The yellow luminescence (YL) band, related to transitions via the −/0 level of the same defect, has a maximum at 2.1 eV. Another yellow luminescence band, which has similar shape but peaks at about 2.2 eV, is observed in less pure GaN samples and is attributed to the CNON complex. In semi-insulating GaN, the GL2 band with a maximum at 2.35 eV (attributed to VN) and the BL2 band with a maximum at 3.0 eV and the ZPL at 3.33 eV (attributed to a defect complex involving hydrogen) are observed. We also conclude that the gallium vacancy-related defects act as centers of nonradiative recombination

Fine structure of the red luminescence band in undoped GaN

Many point defects in GaN responsible for broad photoluminescence (PL) bands remain unidentified. Their presence in thick GaN layers grown by hydride vapor phase epitaxy (HVPE) detrimentally affects the material quality and may hinder the use of GaN in high-power electronic devices. One of the main PL bands in HVPE-grown GaN is the red luminescence (RL) band with a maximum at 1.8 eV. We observed the fine structure of this band with a zero-phonon line (ZPL) at 2.36 eV, which may help to identify the related defect. The shift of the ZPL with excitation intensity and the temperature-related transformation of the RL band fine structure indicate that the RL band is caused by transitions from a shallow donor (at low temperature) or from the conduction band (above 50 K) to an unknown deep acceptor having an energy level 1.130 eV above the valence band

Carbon defects as sources of the green and yellow luminescence bands in undoped GaN

In high-purity GaN grown by hydride vapor phase epitaxy, the commonly observed yellow luminescence (YL) band gives way to a green luminescence (GL) band at high excitation intensity. We propose that the GL band with a maximum at 2.4 eV is caused by transitions of electrons from the conduction band to the 0/+ level of the isolated CN defect. The YL band, related to transitions via the −/0 level of the same defect, has a maximum at 2.1 eV and can be observed only for some high-purity samples. However, in less pure GaN samples, where no GL band is observed, another YL band with a maximum at 2.2 eV dominates the photoluminescence spectrum. The latter is attributed to the CNON complex

Структурные, электрические и люминесцентные характеристики ультрафиолетовых светодиодов, выращенных методом хлорид–гидридной эпитаксии

Electrical and luminescent properties of near−UV light emitting diode structures (LEDs) prepared by hydride vapor phase epitaxy (HVPE) were studied. Variations in photoluminescence and electroluminescence efficiency observed for LEDs grown under nominally similar conditions could be attributed to the difference in the structural quality (dislocation density, density of dislocations agglomerates) of the GaN active layers, to the difference in strain relaxation achieved by growth of AlGaN/AlGaN superlattice and to the presence of current leakage channels in current confining AlGaN layers of the double heterostructure.Изучены электрические и люминесцентные характеристики светодиодных структур (СД), излучающих в ближней ультрафиолетовой (УФ) области и выращенных методом хлорид−гидридной эпитаксии. Обнаружены различия в характеристиках УФ СД, выращенных в номинально одинаковых условиях, которые приписывают различиям в структурном совершенстве (плотности дислокаций и дислокационных агломератов) в активных слоях GaN, разнице в степени релаксации напряжений, достигаемой с помощью сверхрешеток AlGaN/AlGaN, а также существованию каналов токовых утечек в слоях AlGaN, ограничивающих заряд в двойной гетероструктуре.

ФОТОЭЛЕКТРИЧЕСКИЕ ПРЕОБРАЗОВАТЕЛИ В СИСТЕМЕ СО СПЕКТРАЛЬНЫМ РАСЩЕПЛЕНИЕМСОЛНЕЧНОЙ ЭНЕРГИИ

This paper presents results on the simulation of photo converters in a spectral splitting system where solar radiation is separated into three spectral ranges (∆λ1<500 nm, ∆λ2 = 500−725 nm and ∆λ3>725 nm) by means of dichroic filters and then converted to electrical energy by photoconverters based on InGaN/GaN, GaAs/AlGaAs single−junction heterostructures and monocrystalline silicon c−Si. Special attention is paid to the absorption spectrum spreading due to more efficient conversion of the ultraviolet part of the spectrum. The total efficiency of the system varies from 21% to 37% depending on the design of heterostructures.Представлены результаты моделирования фотоэлектрических преобразователей в системе со спектральным расщеплением солнечной энергии, в которой солнечное излучение разделяется с помощью дихроичных фильтров на три спектральных диапазона (∆λ1 < 500 нм, ∆λ2 = 500÷725 нм, ∆λ3 > 725 нм) и затем преобразуется в электроэнергию фотоэлектрическими преобразователями на основе однопереходных гетероструктур InGaN/GaN, GaAs/AlGaAs и монокристаллического кремния c−Si. Особое внимание уделено исследованию расширения спектрального диапазона поглощения системы за счет более эффективного преобразования ультрафиолетовой части спектра. Суммарный КПД системы на всем спектре варьируется от 21 до 37 % в зависимости от дизайна гетероструктур однопереходных фотоэлектрических пре-образователей и вариантов оптических систем

НОВЫЕ НАПРАВЛЕНИЯ РАЗВИТИЯ ТЕХНОЛОГИИ ПРОИЗВОДСТВА УЛЬТРАФИОЛЕТОВЫХ СВЕТОДИОДОВ

The paper presents results of the development of ultraviolet light−emitting diodes based on GaN/AlGaN heterostructures grown on AlN substrates by chloride−hydride vapor phase epitaxy. The peak wavelengths are in the range of 360—365 nm with a spectral width of 10—13 nm; the output optical power of LED dies is 50 mW at 350 mA.Представлены результаты по созданию ультрафиолетовых светодиодов на основе гетеро-структур GaN/AlGaN, выращенных на подложках AlN методом хлоридно-гидридной эпитаксии. Пиковые длины волн находятся в диапазоне 360—365 нм, ширина спектральной кривой составляет 10—13 нм, выходная оптическая мощность чипов светодиодов — 50 мВт при токе 350 мА