1.3-μm InAsP modulation-doped MQW lasers

The effect of both n-type and p-type modulation doping on multiple-quantum-well (MQW) laser performances was studied using gas-source molecular beam epitaxy (MBE) with the object of the further improvement of long-wavelength strained MQW lasers. The obtained threshold current density was as low as 250 A/cm² for 1200-μm-long devices in n-type modulation-doped MQW (MD-MQW) lasers. A very low CW threshold current of 0.9 mA was obtained in 1.3-μm InAsP n-type MD-MQW lasers at room temperature, which is the lowest ever reported for long-wavelength lasers using n-type modulation doping, and the lowest value for lasers grown by all kinds of MBE in the long-wavelength region. Both a reduction of the threshold current and the carrier lifetime in n-type MD MQW lasers caused the reduction of the turn-on delay time by about 30%. The 1.3-μm InAsP strained MQW lasers using n-type modulation doping with very low power consumption and small turn-on delay time are very attractive for laser array applications in high-density parallel optical interconnection systems. On the other hand, the differential gain was confirmed to increase by a factor of 1.34 for p-type MD MQW lasers (N_A=5×10¹⁸ cm ^-3) as compared with undoped MQW lasers, and the turn-on delay time was reduced by about 20% as compared with undoped MQW lasers. These results indicate that p-type modulation doping is suitable for high-speed lasers     

Doping level and type of GaInP saturable absorbing layers forrealizing pulsating 650-nm-band AlGaInP laser diodes

On AlGaInP laser diodes, the doping level and type of GaInP saturable absorbing (SA) layers suitable for self sustained pulsation are clarified. Optical properties of n- and p-type GaInP quantum wells (QWs) have been evaluated by means of time-resolved photoluminescence (TRPL) spectroscopy. As the doping level becomes higher, the recombination lifetime becomes shorter, and it can be reduced to 1.1 ns at our highest doping level (1=2×10¹⁸ cm^-3). For highly doped n-type QW, a PL peak energy shift as large as 26 meV is observed by high-density excitation. Highly doped p-type SA layer is suitable for self-sustained pulsating laser diodes, because it offers short recombination lifetime and no Burstein shift under highly excited condition     

Suppression of the intensity noise in a diode-pumped neodymium:YAGnonplanar ring laser

We investigate the intensity noise properties of a continuous-wave diode pumped Nd:YAG ring-laser system and present results for an active feedback loop that suppresses the relaxation oscillation noise. This system reduces the intensity noise to within 6.1 dB of the quantum noise equivalent level (which is at 1.5×10^-8/√Hz for 1.5 mA) for frequencies between 10 kHz to 300 kHz and to less than 1×10^-7/√Hz for frequencies between 300 Hz and 10 kHz. The technical properties of the optimized feedback system are presented. The theoretical limits of performance for the system are discussed and it is shown that the performance is within 3.1 dB of these limits. We also present data from an optical beat experiment demonstrating that the intensity control system does not introduce any new features into the frequency noise spectrum     

Effect of exponentially graded base doping on the performance ofGaAs/AlGaAs heterojunction bipolar transistors

The authors have fabricated n-p-n GaAs/AlGaAs heterojunction bipolar transistors (HBTs) with base doping graded exponentially from 5×10¹⁹ cm^-3 at the emitter edge to 5×10¹⁸ cm^-3 at the collector edge. The built-in field due to the exponentially graded doping profile significantly reduces base transit time, despite bandgap narrowing associated with high base doping. Compared to devices with the same base thickness and uniform base doping of 1×10¹⁹ cm^-3 , the cutoff frequency is increased from 22 to 31 GHz and maximum frequency of oscillation is increased from 40 to 58 GHz. Exponentially graded base doping also results ill consistently higher common-emitter current gain than uniform base doping, even though the Gummel number is twice as high and the base resistance is reduced by 40%     

Experimental studies of proton-implanted GaAs-AlGaAsmultiple-quantum-well modulators for low-photocurrent applications

We describe the first attempts to control photocurrent, and thus power dissipation, in surface-normal multiple-quantum-well (MQW) modulators. We have made detailed experimental studies of proton-implanted p-i-n GaAs-Al_xGa_1-xAs MQW modulators having barrier layers of x=0.3, 0.45, and 1.0. Structures were implanted to levels of 1×10¹² cm^-2, 1×10¹³ cm^-2, and 1×10¹⁴ cm ^-2. Photocurrent progressively decreased with increasing implant-dose and barrier mole fraction (x). Exciton linewidths showed a strong voltage and implant dose dependence, demonstrating a tradeoff between photocurrent and modulation performance. We obtained our best results with x=1.0 barriers. For example, 1×10¹³ cm^-2-implanted asymmetric Fabry-Perot modulators were realized in which the optical performance was similar to that of unimplanted devices. The photocurrent responsivity was, however, only 0.007 A/W at 12.5 V bias. We report measurements of carrier lifetime in these materials that show the reduction in photocurrent arises from a reduction in lifetime due to implant-induced damage. In addition, the reduced lifetime decreases the optically-excited quantum-well carrier population, leading to an increase in cw saturation intensity. Specifically, 1×10¹³ cm^-2-implanted devices with x=1.0 have a saturation intensity of roughly 45 kW/cm², while unimplanted devices have 3.5 kW/cm². Asymmetric self electro-optic effect devices (A-SEED's) are demonstrated, and power dissipation issues associated with the use of low-photocurrent modulators in integrated systems are discussed     

Minority-carrier transport parameters in degenerate n-type silicon

Direct measurements of the minority-hole transport parameters in degenerate n-type silicon were done by analyzing transient photocurrent in the frequency domain. Minority-hole mobility is found to increase with doping for dopings larger than 4×10¹⁹ cm^-3 . The ratio of minority-hole to majority-hole mobility is found to be about 2.8 at N_D=7.2×10¹⁹ cm^-3. The measured lifetime shows a strongly Auger-dependent mechanism. The extracted Auger coefficient at 296 K is C_n =2.22×10^-31 cm⁶-s^-1, and is in agreement with that reported on other works. Self-consistent checking is used to validate the accuracy of the measured results     

Characterization of highly doped n- and p-type 6H-SiCpiezoresistors

Highly doped (~2×10¹⁹ cm^-3) n- and p-type 6H-SiC strain sensing mesa resistors configured in Wheatstone bridge integrated beam transducers were investigated to characterize the piezoresistive and electrical properties. Longitudinal and transverse gauge factors, temperature dependence of resistance, gauge factor (GF), and bridge output voltage were evaluated. For the n-type net doping level of 2×10¹⁹ cm^-3 the bridge gauge factor was found to be 15 at room temperature and 8 at 250°C. For this doping level, a TCR of -0.24%/°C and -0.74%/°C at 100°C was obtained for the n- and p-type, respectively. At 250°C, the TCR was -0.14%/°C and -0.34%/°C, respectively. In both types, for the given doping level, impurity scattering is implied to be the dominant scattering mechanism. The results from this investigation further strengthen the viability of 6H-SiC as a piezoresistive pressure sensor for high-temperature applications     

Drift hole mobility in strained and unstrained doped Si1-xGex alloys

Results of the drift hole mobility in strained and unstrained SiGe alloys are reported for Ge fractions varying from 0 to 30% and doping levels of 10¹⁵-10¹⁹ cm^-3. The mobilities are calculated taking into account acoustic, optical, alloy, and ionized-impurity scattering. The mobilities are then compared with experimental results for a boron doping concentration of 2×10¹⁹ cm^-3. Good agreement between experimental and theoretical values is obtained. The results show an increase in the mobility relative to that of silicon     

High performance single frequency fiber grating-basederbium/ytterbium-codoped fiber lasers

The device characteristics of Er³⁺,Yb³⁺ single frequency fiber lasers are reported. A 5-cm long 1550-nm distributed feedback fiber laser with 4 mW output power is shown to have excellent specifications in terms of optical linewidth, signal-to-noise ratio (SNR), relative intensity noise, side-mode suppression and polarization purity. For higher power applications, a 1.5 cm single frequency Er³⁺,Yb³⁺ grating-based fiber laser with 60 mW output power and a net efficiency of 12% is demonstrated     

Nonlinear absorption in n-i-p-i-MQW structures

Absorptive nonlinearity in a GaAs/AlGaAs n-i-p-i-MQW (multiple quantum well) structure consisting of alternating n-AlGaAs, i-GaAs/AlGaAs MQW, and p-AlGaAs layers is investigated. A change in the absorption coefficient of more than 4000/cm is obtained in the i-MQW layer with an extremely low excitation intensity on the order of 1 mW/cm ². The figure of merit for absorptive nonlinearity, σ _ch, defined as the change in the absorption coefficient induced by excitation of an electron-hole pair per unit volume, is experimentally evaluated to be 7×10^-13 cm², which is an order of magnitude larger than that for saturation of excitonic absorption in a conventional MQW structure. This experimental value agrees well with the theoretical estimation, which is calculated assuming an optical nonlinear process     

Homodyne optical phase locking of resonant cavity coupledsemiconductor lasers

Two confocal Fabry-Perot cavity coupled semiconductor laser diodes (CFP-LDs) have been constructed for optical phase-locking experiments. Their FM noise suppression characteristics were calculated and compared with measurements of FM noise using an optical resonator as the optical frequency discriminator (FM noise suppression ratio 39 dB). Spectral linewidth was measured and evaluated, and frequency drift of the heterodyne signal in the time domain (20 kHz/s), was also measured. A simple linearized model of the optical feedback system was used for the calculations. Using two CFP-LDs, homodyne optical phase-locking experiments were performed. The performance of the optical phase-locked loop (OPLL) was evaluated by measuring and calculating the phase error variance. The calculation took into account the actual power spectral density of FM noise of the lasers employed in the OPLL. The phase error variance, considering infinite bandwidth, is 2.26×10^-2 rad². Total phase-locked power concentration ratio of the slave laser in the OPLL was 97.7%     

Simultaneous CW operation of shared angular dispersive element WDMlasers

We show that when more than two channels operate continuous-wave (CW) simultaneously in shared dispersive element wavelength-division-multiplexing (WDM) lasers, χ⁽³⁾ in the passive shared waveguide can cause spontaneous fluctuations in frequency and power. We propose the simple cure of making the shared waveguide extremely short. We obtain a value for the nonlinear-index coefficient of our InGaAsP waveguides of +0.9×10^-12 cm ²/W     

1.3-μm strained MQW-DFB lasers with extremely lowintermodulation distortion for high-speed analog transmission

The intermodulation distortion and the noise characteristics of 1.3-μm strained multiquantum-well distributed feedback (MQW-DFB) lasers have been investigated under the modulation frequency of 1.9 GHz in connection with the device structure. In this study, a strained MQW with strain-compensated layers has been introduced in order to increase in the quantum-well number and well width. This causes increase in the differential gain, resulting in increase of the resonance frequency (FR). The FR as high as 5.1 GHz/mW^1/2 has been obtained which is in good agreement with the theoretical calculation. In addition to the strained MQW structure, a new buried heterostructure entirely grown by MOCVD, named as FSBH (facet selective growth buried heterostructure), has been developed to minimize the leakage current which degrades L-I characteristics at high bias current causing the high distortion. The third-order-intermodulation distortion (IMD3) of -88 dBc and relative intensity noise (RIN) of -152 dB/Hz have been obtained under a two-tone test at 1.9 GHz. This suggests that this newly developed laser is quite suitable for high-speed-subcarrier multiplexing transmission     

Minority-carrier transport parameters in n-type silicon

Minority-carrier diffusion length L, lifetime τ, and diffusion coefficient D in n-type Si are measured at 296 K in the doping range from 10¹⁸ cm^-3 to 7×10¹⁹ cm^-3. The measurement is based on a lateral collection of carriers generated by a spatially uniform light. The distance between the illumination edge and the collection junction is defined by photolithography. This allows simultaneous and independent determination of all transport parameters in the same material. A self-consistency and accuracy check is provided by the relation L ²=Dτ. Details of experimental procedures are described. Empirical best-fit relations for the three parameters are given. The extraction of lifetime and diffusion coefficient was done in the frequency domain, which allows for straightforward elimination of parasitic effects in the nanosecond and subnanosecond range     

Design study of AlGaAs/GaAs HBTs

The frequency performance of AlGaAs/GaAs heterojunction bipolar transistors (HBTs) having different layouts, doping profiles, and layer thicknesses was assessed using the BIPOLE computer program. The optimized design of HBTs was studied, and the high current performances of HBTs and polysilicon emitter transistors were compared. It is shown that no current crowding effect occurs at current densities less than 1×10⁵ A/cm² for the HBT with emitter stripe width S_E<3 μm, and the HBT current-handling capability determined by the peak current-gain cutoff frequency is more than twice as large as that of the polysilicon emitter transistor. An optimized maximum oscillation frequency formula has been obtained for a typical process n-p-n AlGaAs/GaAs HBT having base doping of 1×10 ¹⁹ cm^-3     

Auger recombination rates in compressively strainedInxGa1-xAs/InGaAsP/InP(0.53⩽x⩽0.73) multiquantum well lasers

The authors have experimentally determined Auger recombination rates in compressively strained In_xGa_1-xAs/InGaAsP/InP MQW lasers for the first time. The Auger recombination rates were derived from the measured turn-on delay times during large-signal modulation of single-mode lasers. The Auger coefficient increases from 5±1×10^-30 to 13±1×10^-30 cm⁶ s^-1 as the indium composition in the quantum well active region, x, increases from 0.53 to 0.73     

High-performance phase locking of wide linewidth semiconductorlasers by combined use of optical injection locking and opticalphase-lock loop

The requirement for narrow linewidth lasers or short-loop propagation delay makes the realization of optical phase-lock loops using semiconductor lasers difficult. Although optical injection locking can provide low phase error variance for wide linewidth lasers, the locking range is restricted by stability considerations. Theoretical and experimental results for a system which combines both techniques so as to overcome these limitations, the optical injection phase-lock loop (OIPLL), are reported. Phase error variance values as low as 0.006 rad ² (500 MHz bandwidth) and locking ranges exceeding 26 GHz were achieved in homodyne OIPLL systems using DFB lasers of summed linewidth 36 MHz, loop propagation delay of 15 ns and injection ratio less than -30 dB. Phase error variance values as low as 0.003 rad² in a bandwidth of 100 MHz, a mean time to cycle slip of 3×10¹⁰ s and SSB noise density of -94 dBc/Hz at 10 kHz offset were obtained for the same lasers in an heterodyne OIPLL configuration with loop propagation delay of 20 ns and injection ratio of -30 dB     

MBE-grown Si/SiGe HBTs with high β, fT,and fmax

Si/SiGe heterojunction bipolar transistors (HBTs) were fabricated by growing the complete layer structure with molecular beam epitaxy (MBE). The typical base doping of 2×10¹⁹ cm^-3 largely exceeded the emitter impurity level and led to sheet resistances of about 1 kΩ/□. The devices exhibited a 500-V Early voltage and a maximum room-temperature current gain of 550, rising to 13000 at 77 K. Devices built on buried-layer substrates had an f_max of 40 GHz. The transit frequency reached 42 GHz     

High-speed performance of partly gain-coupled 1.55-μmstrained-layer multiple-quantum-well DFB lasers

A partly-gain-coupled 1.55-μm distributed feedback (DFB) laser with a strained-layer multiple-quantum-well (MQW) active region with high relaxation oscillation frequency and maximum intrinsic bandwidth of 28 GHz is reported. An effective differential gain of 1.80×10^-15 cm² was achieved, which may be attributed to the strain effect in the MQW active region as well as the combination of the longitudinal gain/index coupling mechanism and fast lateral carrier injection from the cladding layers into the wells     

N掺杂MgZnO薄膜的p型导电稳定性研究

利用磁控溅射技术,以Mg0.06Zn0.94O为陶瓷靶材,制备了N掺杂p型Mg0.1 3Zn0.8 7O薄膜,薄膜的电阻率为42.45Ω·cm,载流子浓度为3.70×1017/cm3,迁移率为0.40cm2·V-1·s-1。研究了该薄膜p型导电性质在室温空气下随时间的变化情况。实验结果表明,薄膜的电阻率逐渐升高,载流子浓度降低,五个月以后,薄膜转变为n型导电,电阻率为85.58Ω·cm,载流子浓度为4.53×1016/cm3,迁移率为1.61cm2·V-1·s-1。真空热退火后重新转变为p型。结果显示,其p型导电类型的转变与在空气中吸附H2O或H2等形成浅施主有关。