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7th International Conference of Nitride Semiconductors (ICNS-7)

By Earl Franklin,2014-05-07 16:57
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7th International Conference of Nitride Semiconductors (ICNS-7)

    th7 International Conference on Nitride

    Semiconductors (ICNS-7)

     Las Vegas, Nevada, USA

    16 21/09/07

    Kean Boon Lee

    Department of Electronic and Electrical Engineering,

    University of Sheffield

    The ICNS series is a biennial conference which provides a platform for international

    researchers on nitride field to gather and discuss the recent research progress and

    activities on nitride semiconductors. This year, the ICNS-7 was held in Las Vegas, US

    and attracts over 900 participants from different countries. Over 200 oral

    presentations and 400 posters were presented in the conference. The topics covered in

    this conference including material growth, nanostructures, optical/power devices,

    theories, structural analysis and optical/electrical characterisations. In this report, I

    will highlight a few key points from the conference including ultraviolet light emitting

    diodes, visible light emitting diodes, non polar nitride devices and localisation in

    nitride semiconductors.

Ultraviolet light emitting diodes (UV-LEDs):

There are a number of applications of UV light sources including sensing, water/air

    disinfection and bio-medical. A mercury lamp is widely used as UV light source, has

    its disadvantages including its lifetime (500 hours) and it is not environmental

    friendly. Therefore, nitride UV-LED as an UV light source has attracted much

    attention, among those is Professor Asif Khan’s group from University of South Carolina, US, one of the pioneer research teams in nitride UV-LEDs. Professor Khan

    and Dr. Fareed [1-2] talked about the limitation of the 1st generation UV-LEDs. stAlthough 1 generation LEDs technology has been demonstrated to fabricate UV-

    LEDs with wavelength from 338 nm to 254 nm, however, the LEDs have high -10-2threading dislocation density (TDD) (10 cm) and low efficiency (wall plug

    efficiency = 1 %, external quantum efficiency = 3 %).

    p-GaN

    p-AlInGaN p-contact

    AlGaN/AlGaN QW

    n-contact

    n-AlGaN

    Pulsed AlN/GaN

    Superlattice

     Sapphire Figure 1: Schematic device structure of 1stSubstrate generation

    Wavelength (nm) Output power (mW)

    280 1

    254 0.2

    stTable 1: Optical output power of 1 generation UV-LEDs at 20 mA continuous mode

    (cw).

    stBesides, the lifetime of 1 generation LEDs is relatively short compare to visible

    LEDs. The lifetime shows a fast decay and the output fall below 50 % after 1000

    hours. In addition, the output power saturates as injection current increases above 40

    mA due to poor thermal management of the LEDs structure.

    Wavelength (nm) T(hours) output=50%

    280 500-1000

    265 500-700

    250 30

     Table 2: The lifetime of UV-LEDs output power of UV-LEDs fall below 50% of its

    initial value operating at 25mA continuous mode (cw).

In order to achieve higher efficiency and longer lifetime UV devices, Khan et al. [1-2]

    have developed a new technology to grow the UV-LEDs. This technology uses a

    combination of pulsed atomic layer epitaxy (PALE) lateral epitaxial overgrowth and

    hybrid growth of MOCVD and HVPE (MOHVPE) with growth rate up to 10 μm/hr.

    PALE lateral epitaxial overgrowth technique involves growing maskless AlN on

    shallow-grooved (2 μm pillar and 4 μm opening layer) sapphire substrate. This

    technique provides an AlN template which gives a two order of magnitude of TDD

    reduction compared to 1st generation technology. A thick AlGaN layer was then

    grown on the PALE AlN template by HVPE before the n-AlGaN and active region for

    the LEDs. This thick HVPE AlGaN layer is crucial for improved lifetime for the

    device due to lower thermal impedance.

    p-GaN

    p-AlInGaN

    AlGaN/AlGaN QW

    n-AlGaN

    AlGaN (HVPE)

    PALE AlN

     Sapphire Substrate

    Figure 2: UV-LEDs structure grown using hybrid MOHVPE and PALE technology

The performance of the fabricated UV-LEDs using this technology is improved. The

    output power of 280 nm LEDs is 1.6 mW at 20 mA compared to 1

    st generation LEDs which is 1 mW at 20 mA. The improvement of optical power is attributed to lower

    TDD due to PALE AlN template. Besides, no saturation on L-I due to heating effect

was observed below 75 mA under cw showing that better thermal management is

    achieved.

Fujikawa et al [3] reported an approach to improve the UV-LEDs by using p-

    AlInGaN and p-InGaN. They reported the optical power of 346 nm AlInGaN QWs

    LEDs can be increased to 6.6 mW using p-AlInGaN layer compared to the LEDs with

    p-AlGaN layer (1.4 mW). They attribute the improvement of power was due to higher

    hole concentration in p-AlInGaN although they did not present the details of hole

    concentration of their p-AlInGaN layer. They also found that the material quality of

    the p-AlInGaN depends on the growth pressure. For growth pressure below 100 Torr,

    the impurity of p-AlInGaN (oxygen and carbon) was found lower and the hole density

    was higher compared to the p-AlInGaN with growth pressure between 100 to 150

    Torr. Another approach presented by Fujika et al. to improve the output power LEDs

    was to add small amount of indium (4 %) into the top p-GaN layer. The optical power

    of the LEDs with p-InGaN layer is enhanced by factor of 2.

Visible LEDs:

Visible nitride LEDs have been demonstrated to be high efficient, low power

    consumption, long lifetime and robust light source. The superior performance of

    nitride semiconductor makes nitride LEDs an excellent candidate as solid state

    lighting to replace conventional light bulbs and fluorescent lamps. However, it is

    found that the external quantum efficiency (EQE) of visible LEDs depends on the

    emission wavelength. Most of the LEDs’ EQE peaks at around 450 nm and falls

    quickly as the emission wavelength approaches 500 nm. Besides, the efficiency of

    nitride LEDs is also dependent on injection current and always exhibits an efficiency

    droop at higher injection current. The reason causing this efficiency droop remains

    unknown. Many works have been carried to understand the efficiency droop effect in

    order to obtain high power visible LEDs.

Philips Lumileds [4-6] has reported their recent progress in achieving high power

    green LEDs. Their reported they have achieved 115 lm/W for 1 mm2 devices and 131 2lm/W for 2 mm at 350 mA. The extraction efficiency is 80 %, internal quantum

    efficiency is 45 % and 36 % of EQE. The most challenging problem occurs in the

    visible InGaN LEDs is the efficiency droop (EQE drops more than 50 %) at higher

    2injection current (peak efficiency at 1-10 A/cm). This efficiency droop is attributed

    to Auger recombination. The Auger recombination is dependent on carrier density (α 3n), therefore it is expected to be high in QW structure due to high carrier density -306(Auger co-efficient is estimated to be 2x10 cm/s for InGaN). In addition, having

    multiple QWs as an active region will not solve the carrier density problem because

    the heavy holes in nitride material tend to accumulate at the QW close to the p-type

    layer. Therefore, most of the recombination occur at the top QWs which are close to

    the p-layer. In order to reduce carrier density, a thick double hetero-structure (DH)

    (with thickness ~10 nm) InGaN LEDs were proposed and fabricated to replace QWs

    LEDs. It was shown that the injection current for the DH LEDs to reach peak

    efficiency has increased by factor of 10 compare to QW LEDs. However, they did not

    show the efficiency comparison between the DH LEDs and QWs LEDs.

    Many InGaN LEDs show an anomalous intensity collapse in the electroluminescent

    (EL) intensity at low temperature (typically between 100 K to 250 K). The causes of

    this trend remain to be fully explained. The similar behaviour has been observed for

    Sheffield 340 nm AlGaN/GaN SQW LEDs.[7] Fujiwara et al [8] from Kyushu

    Institute of Technology, Japan have studied the temperature dependent EL intensity of

    blue and green InGaN/GaN QWs LEDs from 20 K to 300 K. Their results show that

    blue LEDs have a strong EL intensity collapse at low temperature but the green LEDs

    do not show such behaviour. They attribute this intensity collapse behaviour to carrier

    confinement in the QW at low temperature and in other word, the barrier height of the

    QWs. As the In content in QW is higher for green LEDs, the carriers are more

    confined in the QW and harder to escape/tunnel from the QW due to higher barrier

    height at low temperature. More works are needed to understand this anomalous

    behaviour including the role of electron blocking layer in blocking holes from

    reaching QWs and causes the intensity collapse at low temperature. Non-polar devices:

    Non-polar nitride semiconductors is one of the hot topics in ICNS-7. There are a

    number of advantages of optical devices grown on non-polar orientation (a and m-

    plane) including polarised light output, high quantum efficiency and reduced blue

    shifting with injection current due to absent of built-in field. However, non-polar

    5semiconductor devices suffer from high stacking faults along c-direction (up to 10 -1cm). Okamoto et al [9] presented the performance of their non-polar LEDs and laser diodes (LDs). They reported that their 405 nm InGaN QW LEDs grown on m-plane

    GaN substrate have an optical output of 4 mW and EQE of 7 % at 20 mA. Small

    blueshift (less than 3 nm) is observed from the LEDs as the injection current increases.

    They have successfully fabricated the 401 nm LDs which are stacking-fault free and

    have a flat-surface with 1.5 μm ridge width and cavity length of 600 μm. They

    compared and found that LD stripe on c-axis performs better than a-axis stripe for m-

    plane LDs due to strong anisotropy of photon emission. The current threshold is 20

    mA for c-axis stripe compared to 61 mA for a-axis stripe under pulsed mode. The

    output power is 10 mW at 55 mA. They also fabricated 452 nm LDs but found that

    current threshold current density increases with emission wavelength. For 452 nm

    LDs, the threshold current density is 40 kA/cm22 compared to 10 kA/cm (400 nm

    LDs).

    Schmidt et al [10] from University of California, Santa Barbara also reported non-

    polar InGaN LDs grown on m-plane GaN substrate. They explored the performance

    of LDs without AlGaN cladding layer which is commonly used in c-plane LDs for

    optical wave-guiding. They found that good optical confinement can also achieved by

    having thicker QWs (8 nm) without the AlGaN cladding layer. The benefits of this

    AlGaN-free structure include the structure does not suffer from tensile strain (crack

    formation) and lower operating voltage can be achieved for the device. Indeed, the

    performance of AlGaN cladding-free m-plane LDs was shown to be more superior to

    those with AlGaN clad layer. The threshold current density of AlGaN-free LDs is 2.3

    kA/cm22 compare to 7.5 kA/cm (with AlGaN cladding layer). Besides, the linewidth

    of AlGaN-free LDs (0.4 nm) was also found narrower than the LDs with AlGaN layer

    (1 nm).

Finally, the first semipolar (10-1-1) InGaN LDs grown on GaN substrate has been

    reported by Tyagi et al [11] from University of California, Santa Barbara. The

    structure was grown using MOCVD. The emission wavelength of the semipolar laser

    2)

    compare to non-polar or c-plane laser. The output power is 65 mW at 1.5 A under

    pulsed mode. with 5 μm ridge and 800 μm cavity length is 406 nm with narrow linewidth (less than 0.3 nm). However, the threshold current density is relatively high (16.5 kA/cm

    Localisation in nitride semiconductors:

There is always a question why In-related alloy nitride optical devices have high -10-2optical efficiency despite highly defective (TDD up to 10cm). Does carrier

    localisation play an important role on the high efficiency in the optical devices? If yes,

    what causes carrier localisation and prevents the carriers from diffuse to non-radiative

    recombination centre? Does Al-related alloy exhibit the same effect? There were a lot

    of discussions on carrier localisation in the conference.

Professor Chichibu [12] investigated the origin of defect insensitive emission

    probability in (Al,In,Ga)N alloys. He used positron diffusion and time resolved PL

    technique to study the trapping centres in the AlInN, InGaN, AlInGaN and AlGaN.

    He found that In-containing alloys have large S parameter (related to

    concentration/size of group III vacancy complexes present in the material), short

    positron diffusion length (less than 4 nm) and short radiative lifetime/long non-

    radiative lifetime, attributable to positron trapping in some localised radiative centres.

    On the other hand, unlike In-alloys, AlGaN shows an opposite result, indicating that

    group III vacancy associated defect complexes which act as non-radiative

    recombination centre are high in AlGaN and causes low efficiency in AlGaN

    especially with high Al content. He concluded that carriers localisation in In-related

    alloys is caused by captures of holes in the localised valance states which formed by

    atomic condensates of In-N.

Dawson et al. [13] studied the InGaN/GaN single QW using photoluminescence (PL)

    and photoluminescence excitation (PLE) methods. The PL/PLE is found to be

    excitation / detection energy dependent. The PL from the InGaN SQW shows a zero-

    phonon emission at 2.935 eV. The PLE spectrum, on the other hand, gives a series of

    broad (3.607, 3.710, 3.802 and 3.893 eV assigned as LO phonon assisted transitions) and sharp (3.495 and 3.530 eV possibly heavy/light hole and split-off valance band transitions) minima with detection at high energy side of the zero phonon emission.

    The minima, however show a ‘180

    0 out of phase’ as the detection energy switched to

    low energy side of the zero phonon emission. Besides, the zero phonon emission

    energy in PL appears to blue-shift as the excitation energy increases from 3.607 eV

    (resonant’ excitation) to 3.663 eV (non-resonant’ excitation) at low temperature

    region (6 K to 40 K). Based on the results, the authors proposed that distribution of

    the electrons in the localised states in the QW depends on the electrons’ energy

    distribution before they are captured in the QW. This effect certainly has an

    implication to study the degree of localisation from S-shape emission energy in nitride

    materials.

Finally, I would like to thank UKNC for funding part of my trip to ICNS-7.

References:

    [1] Recent Progress in AlInGaN Based Deep Ultraviolet Light Emitting Diodes: Asif

    Khan; University of South Carolina

    [2] A Novel MOHVPE System for the Growth of High-Quality AlxGa1-xN Layers

    for Deep UV Emitters: R.S. Fareed; K. Balakrishnan; V. Adivarahan; Thomas 122; Takayoshi Takano; Yukihiro Kondo; Katona; Asif Khan; University of South Carolina 112Hideki Hirayama; RIKEN; Matsushita Electric Works, Ltd. [3] 340nm-Band High-Power (>7mW) InAlGaN Quantum Well UV-LED Using p-

    [4] High-Power III-Nitride Based Light Emitting Diodes: Progress and Challenges: Type InAlGaN Layers: Sachie Fujikawa

    Werner Goetz; Philips Lumileds Lighting Company

    [5] High-Power LEDs for Illumination Applications: Frank Steranka; Philips

    Lumileds

    [6] High-Power Blue InGaN-GaN Double-Heterostructure Light Emitting Diodes

    with 40% External Quantum Efficiency at 250 A/cm2: Nathan F. Gardner, Gerd O.

    Mueller, Yu-Chen Shen, Gangyi Chen, Satoshi Watanabe, Anneli Munkholm, Werner

    Goetz, Michael R. Krames

    [7] Temperature Dependence Intensity of 340 nm GaN/AlGaN Ultraviolet Light-

    Emitting Diodes: Kean Lee; Peter Parbrook; Tao Wang; Jie Bai; Fabio Ranalli; Qi Wang; Rob Airey; Geoff Hill; University of Sheffield

    [8] Comparative Study of Temperature-Dependent Electroluminescence Efficiency

    in Blue and Green InGaN Multiple-Quantum-Well Diodes: Kenzo Fujiwara;

    Hiroyuki Jimi; Takayuki Inada; Masaji Horiguchi; Akihiro Satake; Kyushu Institute

    of Technology

    [9]1 Progress of Nonpolar m-Plane InGaN/GaN Laser Diodes: Kuniyoshi Okamoto; 1211Hiroaki Ohta; Shigefusa Chichibu; Taketoshi Tanaka; Testuhiro Tanabe; Hidemi 112Takasu; ROHM Company, Ltd.; Tohoku University 1[10] AlGaN-Cladding-Free Nonpolar InGaN/GaN Laser Diodes: Mathew Schmidt; 11121Daniel Feezell; Robert Farrell; Makoto Saito; Kenji Fujito; Daniel Cohen; James 1111Speck; Shuji Nakamura; Steven DenBaars; University of California, Santa Barbara; 2Mitsubishi Chemical Corporation

    [11] InGaN/GaN Laser Diodes on Semipolar (10-1-1) Bulk GaN Substrates: 11111Anurag Tyagi; Hong Zhong; Roy Chung; Daniel Feezell; Makoto Saito; Kenji 21111Fujito; James Speck; Steven DenBaars; Shuji Nakamura; University of California, 2Santa Barbara; Mitsubishi Chemical Corporation

    [12] Origin of Defect-Insensitive Emission Probability in (Al,In,Ga)N Alloy Films

    Containing In: Shigefusa Chichibu; Tohoku University [13] The Effects of Resonant LO phonon Assisted Excitation on the

    1Photoluminescence Spectra of InGaN/GaN Quantum Wells: P. Dawson, N. P. 12221Hylton, M. J. Kappers, C. McAleese and C.J. Humphreys; University of 2Manchester; University of Cambridge

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