在正向电压激发下ZnSe MIS二极管的自由激子发光

研究了最低温度为20~30K时,在正向电压激发下ZnSe MIS二极管的激子发光光谱,在这一温度下,二极管有可能通过足以产生发光的电流.对于利用通常气相技术生长的高纯晶体所制备的二极管,其电致发光几乎完全由Γ8→Γ6自由激子发光的1LO和2LO声子伴线所组成.根据Gross等人的半经典理论,讨论了两个谱带的形状.结果是谱带的宽度和不对称性归结为激子服从Maxwell-Boltzmann分布,其有效激子温度接近于晶格温度.     

ZnSe单晶质量对其高密度激发发光的影响

用经多次提纯ZnSe原料生长的ZnSe单晶在77K和高密度光激发下观测到了激子-激子(Ex-Ex)散射的P带,随激发密度增加,P带强度增加较快。而在通常的原生ZnSe单晶中只能观测到一个与激子-载流子(Ex-e)散射的Es''带,观测不到P带。经熔融锌中热处理的ZnSe单晶,在上述激发条件下,也观测到了P带。而且此带强度随热处理时间增加而增强。实验表明,P带的产生不仅与激发密度和温度有关,而且还与单晶质量有关。     

ZnSe单晶质量对其高密度激发发光的影响

用经多次提纯ZnSe原料生长的ZnSe单晶在77K和高密度光激发下观测到了激子—激子(Ex-Ex)散射的P带,随激发密度增加,P带强度增加较快。而在通常的原生ZnSe单晶中只能观测到一个与激子—载流子(Ex—e)散射的E_s′带,观测不到P带。经熔融锌中热处理的ZnSe单晶,在上述激发条件下,也观测到了P带。而且此带强度随热处理时间增加而增强。实验表明,P带的产生不仅与激发密度和温度有关,而且还与单晶质量有关。     

正向高密度脉冲电流激发下ZnSe MIS二极管中的自由激子发光

本文测量了在77K和正向脉冲电流密度为50～500mA/mm2的激发下,ZnSe MIS二极管的电致发光光谱.首次在高电流密度激发下的ZnSe晶体的电致发光光谱上,观测到自由激子与自由激子间(Ex-Ex)的散射.本中根据自由激子的动能分布,讨论了2LO声子协助的自由激子伴线的形状,发现当激发电流密度增高时,自由激子的有效温度大于晶格温度,这可归结为激子与激子间的非弹性散射.     

Ⅲ-Ⅴ族化合物中氮束缚激子发光的动力学研究

用发光动力学的分析方法,研究了Ⅲ—Ⅴ族化合物中N束缚激子的发光强度与温度的关系,得到的理论公式与实验结果符合得较好。我们的分析指出,由于无辐射能量传递的存在,束缚激子的△J=2跃迁的发光效率低于△J=1跃迁的发光效率,致使低温(T<50K)下束缚激子的发光强度可能随温度的下降而变弱。另外,N杂质对自由激子再俘获的可能性的大小直接影响束缚激子发光的热猝灭过程:束缚激子可能因热离解成自由激子或自由的电子和空穴。我们找到了区分这两类情况的条件。     

气相外延ZnSe单晶膜的自由激子发光

本文在77K和N2激光器3371谱线高密度激发的VPE ZnSe单晶膜上,首次得到了起因于自由激子与自由激子(Ex-Ex)散射的发光谱带(P带),理论拟合了该谱带的形状并讨论了它的发光特性。文中把在选择的VPE ZnSe外延单晶膜中得到P带的起因归结为这些ZnSe外延单晶膜的质量较高。     

浅ZnCdSe/ZnSe量子阱的光泵受激发射

本文主要研究浅ZnCdSe/ZnSe单量子阱在77K温度下的光致发光。在不同激发密度下,讨论了该结构的发光机制,把77K温度下的受激发射归结为是激子-激子散射所引起的。文中还分析了在浅ZnCdSe/ZnSe量子阱中能够实现与激子相关的受激发射的原因。     

不同激发密度下CdS晶体的光致发光和受激发射

本文主要研究在77-111K温度范围内、不同激发密度的N2激光器的337.1nm谱线激发下,激子-激子(Ex-Ex)、激子-载流子(Ex-e)的相互作用和发射一个LO声子(Ex-1LO)、两个LO声子(Ex-2LO)的自由激子的辐射复合行为.并在77K温度下观测到由Ex-Ex发射产生的受激发射.     

GaP:(Bi,N)晶体中束缚激子的辐射复合及能量转移

本文研究了在4.2K—147K温度范围内GaP:(Bi,N)晶体的光致发光光谱的精细结构,积分强度及其随温度的变化,并将它们与GaP:Bi,GaP:N光谱进行比较。首次发现在GaP:(Bi,N)晶体中激发能量从孤立N中心和NNi对(i≥3)中心到Bi中心的转移,增大了Bi束缚激子态的浓度,使Bi发射带增强。     

CdS单晶中的绿色电致发光和光致发光

在50-290K温度范围内,研究了在正向电压激发下,用高纯CdS单晶制备的MIS发光二极管的电致发光光谱。在室温时,发射光谱分别由峰值为5135±25Å和5300±15Å的二个绿色谱带所组成,而在低温下,观测到有很多结构的光谱。在50K时,自由和束缚激子的发射实际上占有统治地位。文中提出,在室温时,谱峰为5135±25Å的谱带是与受导带自由电子散射的自由激子的发射有关;而谱峰为5300±15Å的谱带归结为与同时发射二个纵光学声子的自由激子的发射有关。比较了刚生长的高阻CdS单晶以及在熔融镉中热处理的低阻CdS单晶的光致发光光谱。在熔融镉中的热处理抑制了自由到束缚(HES)和束缚到束缚(LES)的边缘发射,也抑制了I2束缚激子谱线,但是增强了自由激子的发射,在电场激发下,也使自由激子的发射增强。     

高纯CdS单晶生长及光学性质

本文报道用物理蒸气输运(PVT)法,通过对原料的提纯和控制组分蒸气压生长出高纯CdS单晶.叙述了控制晶体化学比的方法并通过比较CdS的激子光谱以评价材料的纯度.     

Photocurrent decay in silicon at low temperatures

We report measurements performed at low temperatures in silicon of the transient photovoltaic current following excitation by an intense laser light pulse. They show the occurrence of a delayed minor current peak, which we suggest is related to the free carriers obtained, when electron-hole droplets after drift in an exciton density gradient are destroyed by heating.     

Short-circuit current improvement in CdTe solar cells by combining a ZnO buffer layer and a solution back contact

Conventional CdTe solar cells have a CdS window layer, in which an absorption loss of photons with more than 2.4 eV occurs through the CdS layer. A thinner CdS layer was applied to enhance light transmission and a ZnO buffer layer with a band gap of 3.3 eV was introduced to suppress shunting through the thinner CdS window layer. A 100-nm thick ZnO layer sputter-deposited at 300 °C had uniform coverage on a transparent conductive oxide (TCO) after a subsequent high-temperature process. The ZnO layer was effective in preventing shunting through the CdS window layer so that the open-circuit voltage and fill factor of the CdTe solar cells were recovered and the short-circuit current was enhanced over that of the conventional CdTe solar cell. In the ZnO/CdS/CdTe configuration, the short-circuit current was further improved throughout the visible wavelength region by replacing the Cu-metal contact with a Cu solution contact. As a result the short-circuit current from 21.7 to 26.1 mA/cm² and the conversion efficiency of the CdTe solar cell increased from 12 to 15% without antireflective coating. Our result indicates that the Cu solution back contact is a critical factor for achieving a higher cell efficiency in addition to ZnO buffer layer.     

Temporal evolution of exciton resonant secondary emission spectrum

The exciton secondary emission spectrum of CdS crystals has been studied at 2 K under $\text{λ =}\text{4765}\text{A}\text{?}\text{Ar}{}^{\mathrm{+}}$ laser excitation in the samples with different exciton lifetime. In these conditions three distinct components of secondary emission — Raman scattering, hot and equilibrium luminescence were spectrally resolved. It has been shown that the exciton lifetime acts like a very fast optical shutter and permits one to observe in stationary conditions the temporal evolution of the exciton secondary emission spectrum. The energy relaxation of excitons is accompanied by its “phase” relaxation, the measure of the latter being the polarization degree of emission. With the aid of the Hanle effect the acoustic phonon relaxation time 3.5 × 10^-12 sec has been estimated. The exciton lifetime has been found to vary from 3 × 10^-12 to 1.6 × 10^-9 sec in the samples studied.     

Nanosecond acoustic strain pulses from CdS phonon masers

Observations of nanosecond strain pulses generated by a mode-locked CdS phonon maser are reported. Peak strains exceeding 5 × 10^-5 have been measured. An analogy is drawn between the observed mode-locked operation and a repetitive pulse generator.     

Exciton-contribution to the reflection spectrum at high excitation intensities in CdS

We report measurements of changes in reflection spectrum of CdS due to increasing the density of photoexcited carriers at temperatures above the critical temperature for electron-hole liquid formation. The contribution of the exciton resonance is seen to decrease and analysis of the lineshape indicates that this decrease is due to exciton-exciton collision and a change in exciton polarizability. These results are consistent with a transition from exciton to free electron-hole plasma (Mott transition) at a density of n ～ 2.5 x 10¹⁷ cm^-3.     

High density excitation effects on fluorescence from the crystals of aromatic hydrocarbons and charge-transfer complexes

High density excitation effects on fluorescence were studied for the crystals of fluoranthene, chrysene, benzo[g, h, i]perylene, pyrene and pyrene-d₁₀ and also for the crystal of 1,2,4,5-tetracyanobenzene (TCNB)-hexamethylbenzene (HMB) complex. Relative fluorescence intensity in the shorter wavelength region decreased with increasing excitation density (for chrysene, fluoranthene, and benzo[g, h, i]perylene), and the vibrational structure became diffuse at high density excitation (for fluoranthene and chrysene). The rate constants of the bimolecular quenching and exciton migration were obtained by analysis of the fluorescence decay curves at high density excitation for chrysene, pyrene, pyrene-d₁₀, and TCNB-HMB. The exciton hopping rates at 295 K were 2.4 × 10¹⁰ s^?1 in chrysene and 7.9 × 10⁸s^?1 in TCNB-HMB. The differences in the rate between chrysene and fluoranthene (2.2 × 10⁹s^?1) and between TCNB-HMB and TCNB-durene (4.2 × 10⁹s^?1) are discussed in terms of the coupling intensity between two neighboring oscillators accompanying the transition between ground and the lowest excited singlet state. The difference in the rate and activation energy of exciton migration between pyrene and pyrene-d₁₀ corresponded to a quasilocalized exciton model.