`
`IN THE UNITED STATES PATENT AND TRADEMARK OFFICE
`
`In re Patent of:
`
`Lebens et al.
`
`U.S. Patent No.:
`
`6,095,661
`
`
`
`Issue Date:
`
`August 1, 2000
`
`Appl. Serial No.:
`
`09/044,559
`
`Filing Date:
`
`March 19, 1998
`
`Title:
`
`METHOD AND APPARATUS FOR AN L.E.D.
`
`FLASHLIGHT
`
`
`PETITION FOR INTER PARTES REVIEW OF UNITED STATES PATENT
`NO. 6,095,661 PURSUANT TO 35 U.S.C. §§ 311–319, 37 C.F.R. § 42
`
`
`Exhibit LG-1009
`
`M. Schauler et al, GaN based LED’s with different recombination zones, MSR
`
`Internet Journal of Nitride Research, Volume 2
`
`Article 44 (“Admitted Prior Art”)
`
`

`

`M R S
`
`Internet Journal o f
`
`Nitride S emiconductor Research
`
`Volume 2, Article 44
`
`GaN based LED's with different recombination zones
`
`M. Schauler, C. Kirchner, M. Mayer, A. Pelzmann, F. Eberhard, Markus Kamp , P. Unger, K. J. Ebeling
`Abteilung Optoelektronik, Universität Ulm
`
`This article was received on June 15, 1997 and accepted on October 8, 1997.
`
`Abstract
`
`GaN based homo- and heterotype LED's have been fabricated and characterized which emit in the blue
`and ultra-violet part of the spectral range. Complete epitaxial LED layer sequences with different
`recombination zones have been grown using MOVPE as well as MBE. Subsequent to the material growth,
`chemically-assisted ion-beam etching and contact metallization are utilized to achieve full LED devices.
`MBE-grown homotype LED's reveal a peak in the output light spectrum at a wavelength of 372 nm with a
`linewidth being as narrow as 12 nm. GaN/InGaN LED's grown by MOVPE show visible single peak emission
`with linewidths of 23 nm. The optical output power as measured in a calibrated Ulbricht sphere is in the 1 m W
`regime.
`
`1. Introduction
`
`In recent times, there has been increasing interest in light-emitting diodes (LED's) which emit in the visible spectral
`range from green to blue [1]. Such devices are key components for LED-based full-color displays that are going to
`become a significant market over the next years. Another interesting application will be the conversion of blue or UV
`radiation from GaN-based LED's into virtually any color by phosphors or organic dyes. Particularly white LED's are
`expected to gain a reasonable share of the illumination market because LED's are smaller, much more robust, and
`live about 50 times longer compared to ordinary light bulbs [2].
`
`For the present work on GaN LED's, two major growth techniques, molecular beam epitaxy (MBE) and metalorganic
`vapor phase epitaxy (MOVPE) are employed. Whereas the MOVPE approach is a rather standard one, the MBE
`approach is somewhat particular, since it uses the ammonia dissociation on the growing surface to provide the atomic
`nitrogen [3]. Processing and characterization of the devices are performed under identical conditions. Differences in
`the device performance can therefore be attributed to the vertical structure of the LED.
`
`We will emphasize the role of a recombination zone in order to obtain single wavelength emission from the devices.
`For this purpose we investigate homojunction LED's without recombination zone (MOVPE), homojunction devices
`with an undoped recombination zone (MBE), and heterotype InGaN/GaN LED's with an InGaN recombination zone
`(MOVPE).
`
`2. Experimental
`
`In the following, we briefly describe the growth process for GaN using either MBE or MOVPE, the processing of the
`structures to full devices, and the characterization methods employed. All GaN structures are grown on c-plane
`oriented sapphire. Both growth methods make use of a low-temperature GaN nucleation layer, where the actual
`parameters of deposition are optimized individually. The structures in general consist of approximately 1 m m of
`n-type GaN topped by 0.5 m m to 1 m m of p-type GaN. For the homotype MBE-grown LED, an additional 50-nm-thin
`undoped recombination zone is inserted between n- and p-type material. The recombination zone for the
`heterotype MOVPE LED consists of a 50-nm-thick In0.15Ga0.85N layer.
`
`For MOVPE growth, a horizontal reactor (AIXTRON AIX 200 RF) is operated at low pressure. Trimethylgallium (TMGa),
`trimethylaluminum (TMAl) and trimethylindium (TMIn) are used as group-III precursors. The group-V element,
`
`Exhibit LG-1009 Page 1
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`

`

`,
`p
`g
`p
`p
`g
`)
`(
`y
`)
`(
`y
`nitrogen, is delivered by ammonia and hydrogen is used as carrier gas. n- and p-doping are achieved by the use of
`silane (SiH4) and bis-cyclopentadienylmagnesium (Cp2Mg), respectively. The growth of GaN is performed at
`temperatures ranging from 1020 (cid:176) C to 1060 (cid:176) C. After a thermal p-activation step, i.e. annealing in nitrogen ambient
`at 750 (cid:176) C for 15 min, the material is ready for processing.
`
`The MBE samples are grown in an almost standard MBE system (Riber MBE 32) which has been adapted to group-V
`gas sources. NH3 is introduced into the growth chamber through a standard high-temperature gas injector (Riber HTI
`432). The decomposition of NH3 is carried out with an on-surface-cracking technique, where the ammonia molecules
`are thermally dissociated on the growing surface [3]. Elemental Ga, Al and In are supplied by effusion cells. The
`crystal itself is grown at a temperature of approximately 720 (cid:176) C. An activation of the acceptors is not necessary.
`
`Identical processing steps are applied to MBE- and MOVPE-grown material. In the first photolithographic step, the
`mesa structure is defined. Using a conventional photoresist mask, the pattern is transferred into the semiconductor
`by chemically-assisted ion-beam etching (CAIBE) thereby creating steep side walls. With gas flows of 6 sccm Ar in
`the ion source and of 4 sccm Cl2 in the ring nozzle above the substrate, an etch rate of 70 nm/min is achieved.
`Subsequently, the samples are boiled in acetone, isopropanol, and methanol to remove all organic depositions
`from the surface. The n-contact areas are defined by the second lithographic step. The contact itself is made by
`lift-off technique and consists of 30-nm-thick Ti strengthened by a 150-nm-thick Au layer. The final photolithographic
`process is the p-contact metallization. For contact formation, 20 nm of Ni and 150 nm of Au are deposited. A
`schematic illustration of the fabrication sequence depicting the different steps of processing is shown in Figure 1.
`After dicing with a diamond saw, the LED's are placed into carriers, bonded, and sealed with a transparent resin.
`
`3. Results
`
`A typical IV characteristics of a LED is shown in Figure 2. Turn-on voltages of the devices are between 2.5 and 3 V;
`breakdown occurs at a reverse voltage of approximately 8 V. A series resistance of 60 W
` can be deduced from the
`IV characteristics, which is comparable to the values of commercially available devices. The specific contact
`resistance is determined using circular transmission-line structures [4]. On p-type GaN with p » 1 ·1017 cm-3, a value
`of 1 ·10-2 W
` cm2 for Ni/Au contacts is achieved. On n-type material with free electron concentrations of about n » 1
`·1019 cm-3, specific contact resistances are in the 10-5 W
` cm2 range for Ti/Au contacts.
`
`Homotype LED's fabricated from MOVPE material feature emission spectra as shown in Figure 3. A relatively broad
`electroluminescence (EL) band with the two emission peaks gives the LED a bluish-white appearance, which
`additionally varies with current since the relative intensities of the peaks change. At lower current densities, the
`maximum intensity occurs at a wavelength of about 440 nm. Such an emission can be attributed to the Mg related
`transitions which is also observable in photoluminescence (PL) spectra at 300 K. At higher current densities Figure
`3 reveals the dominance of the emission at 380 nm. This is in accordance to S. Nakamura’s observations [5] who
`found a similar behavior for his homotype GaN LED's in case of a non-perfect p-doping.
`
`LED's featuring a separate recombination zone behave differently as can be observed in the single peak spectra
`shown in Figure 4 and Figure 5. The MBE-grown homotype LED which contains the above mentioned 50-nm-thick
`undoped recombination zone, reveals a narrow electroluminescence linewidth with a full-width at half-maximum
`(FWHM) value of only 12 nm at a peak wavelength of 372 nm when operated at a driving current of 20 mA ( Figure 4).
`A variation of the current from 25 mA to 40 mA results in a very small shift of the emission peak from 368 nm to 374
`nm. No indication of Mg-induced recombination, neither at 430 nm nor at 380 nm, is observed in the EL spectra.
`Only one radiative transition path is prominent in this structure. From the energetic position, it can be derived that the
`light generation occurs in the undoped GaN region between p- and n-type material. The difference in the peak PL
`emission at a wavelength of 364 nm related to the free exciton transition and the EL emission peak at 368 nm is
`probably due to ohmic heating. Since the light generated in the intrinsic region of the junction is absorbed in the p-
`and n-doped cladding layers, the lineshape reveals a sharp decrease at the high energy side and a broader tail at
`the low energy side of the spectrum which also leads to a reduction in linewidth.
`
`GaN/InGaN/GaN double heterostructure LED's grown by MOVPE reveal emission spectra as shown in Figure 5. The
`device features single peak emission at a wavelength of 392 nm with a FWHM linewidth as small as 23 nm. Even at
`high driving current densities, an intrinsic GaN peak can barely be detected in the spectrum and the Mg-related long
`wavelength tail is not present. The wavelength shift caused by driving current variation is negligible. PL
`measurements on a sample with an InGaN layer embedded in undoped GaN grown under identical conditions show a
`transitions at at an emission wavelength of 401 nm, proving that recombination takes place in the InGaN layer of the
`LED.
`
`Exhibit LG-1009 Page 2
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`

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`A comparison of the EL spectra obtained from the different LED's reveals striking differences which are attributed to
`the vertical structures rather than to the different growth techniques. The standard homotype pn-junction (Figure 3)
`is similar in almost every feature (lineshape, linewidth, current dependence, peak positions, etc.) to results obtained
`by Nakamura [5] for p-doping of about 1 ·1017 cm-3. With introduction of an intrinsic GaN layer (Figure 4), the
`transitions at 440 nm and 380 nm are no longer present, indicating that the electron-hole recombination does not
`occur in the Mg-doped part of the junction, but in the intrinsic region. This leads to single peak emission and a
`significant reduction of the linewidth, however, the structure suffers from internal absorption. Since the observed
`small linewidth of 12 nm is partially caused by the absorption mechanism, this feature can not be compared to results
`obtained with LED's where no internal absorption occurs. Substitution of the intrinsic GaN recombination layer with
`InGaN provides an improved carrier confinement and avoids internal absorption (Figure 5).
`
`The optical output power of the LED's has been measured with a calibrated Ulbricht sphere. For the MOVPE-grown
`homojunction LED ( Figure 3) a saturation occurs at a power level of about 0.7 m W and a corresponding driving
`current of 30 mA. Under comparable conditions we do not observe a saturation for the MBE homojunction with
`recombination layer ( Figure 4). However, the output power is lower (0.4 m W at 25 mA) because a significant part of
`the generated light is absorbed in the cladding layers.
`
`Lifetime measurements have been performed with the MOVPE-grown homotype LED by monitoring the optical
`output power at a constant driving current versus time. A 1 db loss in output power is a commonly used failure
`criterion. Assuming an exponential decrease in output power over time, the described LED passes industrial
`requirements for more than 4500 h of continuous operation at room temperature.
`
`4. Summary
`
`GaN LED's with different recombination zones have been fabricated using MOVPE as well as MBE growth.
`Emission spectra of homojunction LED's show a shift in the peak emission wavelength with operating current that
`can be attributed to a second recombination path predominant at high current densities. MBE-grown homotype
`LED's featuring an undoped GaN layer as recombination zone exhibit a narrow emission linewidth (12 nm) at a single
`wavelength. Therefore, they are suitable pump-light sources for luminescence-converting devices using phosphors
`or organic dyes. MOVPE double-heterostructure LED's show single peak emission from an InGaN recombination
`zone. Emission at 392 nm with a narrow linewidth (FWHM 23 nm) is demonstrated for this InGaN/GaN LED. The
`optical output power of the devices is in the 1 m W regime and is still subject to improvement.
`
`References
`
`[1] Shuji Nakamura, Takashi Mukai, Masayuki Senoh , Appl. Phys. Lett. 64, 1687-1689 (1994).
`
`[2] P Schlotter, R Schmitt, J Schneider, Appl. Phys. A 64, 417-418 (1997).
`
`[3] M. Kamp, M. Mayer, A. Pelzmann, K. J. Ebeling, Mater. Res. Soc. Symp. Proc. 449, 161-172 (1997).
`
`[4] GK Reeves, Sol. St. Electr. 23, 487-490 (1980).
`
`[5] S. Nakamura, T. Mukai, M. Senoh, Jpn. J. Appl. Phys. 30, L1998 (1991).
`
`Exhibit LG-1009 Page 3
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`

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`Figure 1. Fabrication steps for GaN-based LED's:
`(a) after epitaxial growth, (b) after mesa etching
`using chemically-assisted ion-beam etching, (c)
`after n-contact metallization, and (d) after p-contact
`metallization.
`
`Figure 2. IV characteristic of a homotype
`MOVPE-grown LED. Typical devices exhibit
`turn-on volages between 2.5 and 3 V, breakdown
`voltages of approximately 8 V, and series
`resistances of 60 W
`.
`
`Figure 3. Electroluminescence spectra of a
`homotype MOVPE-grown LED at different driving
`currents. Relatively broad electroluminescence
`ranges are observed. The peak emission at a
`wavelength of 440 nm (Mg related transitions) for
`low driving currents shift towards 380 nm for high
`current densities.
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`Exhibit LG-1009 Page 4
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`

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`Figure 4. Electroluminescence spectra of a
`homotype MBE-grown LED featuring an undoped
`recombination layer at different driving currents. A
`FWHM linewidth of only 12 nm at a peak
`wavelength of 372 nm is observed.
`
`Figure 5. Electroluminescence spectra of a
`heterotype MOVPE-grown InGaN/GaN LED at
`different driving currents. Due to the
`heterostructure, the emission peak is shifted
`towards longer wavelengths (392 nm) having a
`FWHM linewidth of 23 nm.
`
`© 1997 The Materials Research Society
`
`M R S
`
`Internet Journal o f
`
`Nitride S emiconductor Research
`
`Exhibit LG-1009 Page 5
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`